presentation layer addressing

The OSI Model – The 7 Layers of Networking Explained in Plain English

This article explains the Open Systems Interconnection (OSI) model and the 7 layers of networking, in plain English.

The OSI model is a conceptual framework that is used to describe how a network functions. In plain English, the OSI model helped standardize the way computer systems send information to each other.

Learning networking is a bit like learning a language - there are lots of standards and then some exceptions. Therefore, it’s important to really understand that the OSI model is not a set of rules. It is a tool for understanding how networks function.

Once you learn the OSI model, you will be able to further understand and appreciate this glorious entity we call the Internet, as well as be able to troubleshoot networking issues with greater fluency and ease.

All hail the Internet!

Prerequisites

You don’t need any prior programming or networking experience to understand this article. However, you will need:

• Basic familiarity with common networking terms (explained below)
• A curiosity about how things work :)

Learning Objectives

Over the course of this article, you will learn:

• What the OSI model is
• The purpose of each of the 7 layers
• The problems that can happen at each of the 7 layers
• The difference between TCP/IP model and the OSI model

Common Networking Terms

Here are some common networking terms that you should be familiar with to get the most out of this article. I’ll use these terms when I talk about OSI layers next.

A node is a physical electronic device hooked up to a network, for example a computer, printer, router, and so on. If set up properly, a node is capable of sending and/or receiving information over a network.

Nodes may be set up adjacent to one other, wherein Node A can connect directly to Node B, or there may be an intermediate node, like a switch or a router, set up between Node A and Node B.

Typically, routers connect networks to the Internet and switches operate within a network to facilitate intra-network communication. Learn more about hub vs. switch vs. router.

Here's an example:

For the nitpicky among us (yep, I see you), host is another term that you will encounter in networking. I will define a host as a type of node that requires an IP address. All hosts are nodes, but not all nodes are hosts. Please Tweet angrily at me if you disagree.

Links connect nodes on a network. Links can be wired, like Ethernet, or cable-free, like WiFi.

Links to can either be point-to-point, where Node A is connected to Node B, or multipoint, where Node A is connected to Node B and Node C.

When we’re talking about information being transmitted, this may also be described as a one-to-one vs. a one-to-many relationship.

A protocol is a mutually agreed upon set of rules that allows two nodes on a network to exchange data.

“A protocol defines the rules governing the syntax (what can be communicated), semantics (how it can be communicated), and synchronization (when and at what speed it can be communicated) of the communications procedure. Protocols can be implemented on hardware, software, or a combination of both. Protocols can be created by anyone, but the most widely adopted protocols are based on standards.” - The Illustrated Network.

Both wired and cable-free links can have protocols.

While anyone can create a protocol, the most widely adopted protocols are often based on standards published by Internet organizations such as the Internet Engineering Task Force (IETF).

A network is a general term for a group of computers, printers, or any other device that wants to share data.

Network types include LAN, HAN, CAN, MAN, WAN, BAN, or VPN. Think I’m just randomly rhyming things with the word can ? I can ’t say I am - these are all real network types. Learn more here .

Topology describes how nodes and links fit together in a network configuration, often depicted in a diagram. Here are some common network topology types:

A network consists of nodes, links between nodes, and protocols that govern data transmission between nodes.

At whatever scale and complexity networks get to, you will understand what’s happening in all computer networks by learning the OSI model and 7 layers of networking.

What is the OSI Model?

The OSI model consists of 7 layers of networking.

First, what’s a layer?

No, a layer - not a lair . Here there are no dragons.

A layer is a way of categorizing and grouping functionality and behavior on and of a network.

In the OSI model, layers are organized from the most tangible and most physical, to less tangible and less physical but closer to the end user.

Each layer abstracts lower level functionality away until by the time you get to the highest layer. All the details and inner workings of all the other layers are hidden from the end user.

How to remember all the names of the layers? Easy.

• Please | Physical Layer
• Do | Data Link Layer
• Not | Network Layer
• Tell (the) | Transport Layer
• Secret | Session Layer
• Password (to) | Presentation Layer
• Anyone | Application Layer

Keep in mind that while certain technologies, like protocols, may logically “belong to” one layer more than another, not all technologies fit neatly into a single layer in the OSI model. For example, Ethernet, 802.11 (Wifi) and the Address Resolution Protocol (ARP) procedure operate on >1 layer.

The OSI is a model and a tool, not a set of rules.

OSI Layer 1

Layer 1 is the physical layer . There’s a lot of technology in Layer 1 - everything from physical network devices, cabling, to how the cables hook up to the devices. Plus if we don’t need cables, what the signal type and transmission methods are (for example, wireless broadband).

Instead of listing every type of technology in Layer 1, I’ve created broader categories for these technologies. I encourage readers to learn more about each of these categories:

• Nodes (devices) and networking hardware components. Devices include hubs, repeaters, routers, computers, printers, and so on. Hardware components that live inside of these devices include antennas, amplifiers, Network Interface Cards (NICs), and more.
• Device interface mechanics. How and where does a cable connect to a device (cable connector and device socket)? What is the size and shape of the connector, and how many pins does it have? What dictates when a pin is active or inactive?
• Functional and procedural logic. What is the function of each pin in the connector - send or receive? What procedural logic dictates the sequence of events so a node can start to communicate with another node on Layer 2?
• Cabling protocols and specifications. Ethernet (CAT), USB, Digital Subscriber Line (DSL) , and more. Specifications include maximum cable length, modulation techniques, radio specifications, line coding, and bits synchronization (more on that below).
• Cable types. Options include shielded or unshielded twisted pair, untwisted pair, coaxial and so on. Learn more about cable types here .
• Signal type. Baseband is a single bit stream at a time, like a railway track - one-way only. Broadband consists of multiple bit streams at the same time, like a bi-directional highway.
• Signal transmission method (may be wired or cable-free). Options include electrical (Ethernet), light (optical networks, fiber optics), radio waves (802.11 WiFi, a/b/g/n/ac/ax variants or Bluetooth). If cable-free, then also consider frequency: 2.5 GHz vs. 5 GHz. If it’s cabled, consider voltage. If cabled and Ethernet, also consider networking standards like 100BASE-T and related standards.

The data unit on Layer 1 is the bit.

A bit the smallest unit of transmittable digital information. Bits are binary, so either a 0 or a 1. Bytes, consisting of 8 bits, are used to represent single characters, like a letter, numeral, or symbol.

Bits are sent to and from hardware devices in accordance with the supported data rate (transmission rate, in number of bits per second or millisecond) and are synchronized so the number of bits sent and received per unit of time remains consistent (this is called bit synchronization). The way bits are transmitted depends on the signal transmission method.

Nodes can send, receive, or send and receive bits. If they can only do one, then the node uses a simplex mode. If they can do both, then the node uses a duplex mode. If a node can send and receive at the same time, it’s full-duplex – if not, it’s just half-duplex.

The original Ethernet was half-duplex. Full-duplex Ethernet is an option now, given the right equipment.

How to Troubleshoot OSI Layer 1 Problems

Here are some Layer 1 problems to watch out for:

• Defunct cables, for example damaged wires or broken connectors
• Broken hardware network devices, for example damaged circuits
• Stuff being unplugged (...we’ve all been there)

If there are issues in Layer 1, anything beyond Layer 1 will not function properly.

Layer 1 contains the infrastructure that makes communication on networks possible.

It defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating physical links between network devices. - Source

Fun fact: deep-sea communications cables transmit data around the world. This map will blow your mind: https://www.submarinecablemap.com/

And because you made it this far, here’s a koala:

OSI Layer 2

Layer 2 is the data link layer . Layer 2 defines how data is formatted for transmission, how much data can flow between nodes, for how long, and what to do when errors are detected in this flow.

In more official tech terms:

• Line discipline. Who should talk for how long? How long should nodes be able to transit information for?
• Flow control. How much data should be transmitted?
• Error control - detection and correction . All data transmission methods have potential for errors, from electrical spikes to dirty connectors. Once Layer 2 technologies tell network administrators about an issue on Layer 2 or Layer 1, the system administrator can correct for those errors on subsequent layers. Layer 2 is mostly concerned with error detection, not error correction. ( Source )

There are two distinct sublayers within Layer 2:

• Media Access Control (MAC): the MAC sublayer handles the assignment of a hardware identification number, called a MAC address, that uniquely identifies each device on a network. No two devices should have the same MAC address. The MAC address is assigned at the point of manufacturing. It is automatically recognized by most networks. MAC addresses live on Network Interface Cards (NICs). Switches keep track of all MAC addresses on a network. Learn more about MAC addresses on PC Mag and in this article . Learn more about network switches here .
• Logical Link Control (LLC): the LLC sublayer handles framing addressing and flow control. The speed depends on the link between nodes, for example Ethernet or Wifi.

The data unit on Layer 2 is a frame .

Each frame contains a frame header, body, and a frame trailer:

• Header: typically includes MAC addresses for the source and destination nodes.
• Body: consists of the bits being transmitted.
• Trailer: includes error detection information. When errors are detected, and depending on the implementation or configuration of a network or protocol, frames may be discarded or the error may be reported up to higher layers for further error correction. Examples of error detection mechanisms: Cyclic Redundancy Check (CRC) and Frame Check Sequence (FCS). Learn more about error detection techniques here .

Typically there is a maximum frame size limit, called an Maximum Transmission Unit, MTU. Jumbo frames exceed the standard MTU, learn more about jumbo frames here .

How to Troubleshoot OSI Layer 2 Problems

Here are some Layer 2 problems to watch out for:

• All the problems that can occur on Layer 1
• Unsuccessful connections (sessions) between two nodes
• Sessions that are successfully established but intermittently fail
• Frame collisions

The Data Link Layer allows nodes to communicate with each other within a local area network. The foundations of line discipline, flow control, and error control are established in this layer.

OSI Layer 3

Layer 3 is the network layer . This is where we send information between and across networks through the use of routers. Instead of just node-to-node communication, we can now do network-to-network communication.

Routers are the workhorse of Layer 3 - we couldn’t have Layer 3 without them. They move data packets across multiple networks.

Not only do they connect to Internet Service Providers (ISPs) to provide access to the Internet, they also keep track of what’s on its network (remember that switches keep track of all MAC addresses on a network), what other networks it’s connected to, and the different paths for routing data packets across these networks.

Routers store all of this addressing and routing information in routing tables.

Here’s a simple example of a routing table:

The data unit on Layer 3 is the data packet . Typically, each data packet contains a frame plus an IP address information wrapper. In other words, frames are encapsulated by Layer 3 addressing information.

The data being transmitted in a packet is also sometimes called the payload . While each packet has everything it needs to get to its destination, whether or not it makes it there is another story.

Layer 3 transmissions are connectionless, or best effort - they don't do anything but send the traffic where it’s supposed to go. More on data transport protocols on Layer 4.

Once a node is connected to the Internet, it is assigned an Internet Protocol (IP) address, which looks either like 172.16. 254.1 (IPv4 address convention) or like 2001:0db8:85a3:0000:0000:8a2e:0370:7334 (IPv6 address convention). Routers use IP addresses in their routing tables.

IP addresses are associated with the physical node’s MAC address via the Address Resolution Protocol (ARP), which resolves MAC addresses with the node’s corresponding IP address.

ARP is conventionally considered part of Layer 2, but since IP addresses don’t exist until Layer 3, it’s also part of Layer 3.

How to Troubleshoot OSI Layer 3 Problems

Here are some Layer 3 problems to watch out for:

• All the problems that can crop up on previous layers :)
• Faulty or non-functional router or other node
• IP address is incorrectly configured

Many answers to Layer 3 questions will require the use of command-line tools like ping , trace , show ip route , or show ip protocols . Learn more about troubleshooting on layer 1-3 here .

The Network Layer allows nodes to connect to the Internet and send information across different networks.

OSI Layer 4

Layer 4 is the transport layer . This where we dive into the nitty gritty specifics of the connection between two nodes and how information is transmitted between them. It builds on the functions of Layer 2 - line discipline, flow control, and error control.

This layer is also responsible for data packet segmentation, or how data packets are broken up and sent over the network.

Unlike the previous layer, Layer 4 also has an understanding of the whole message, not just the contents of each individual data packet. With this understanding, Layer 4 is able to manage network congestion by not sending all the packets at once.

The data units of Layer 4 go by a few names. For TCP, the data unit is a packet. For UDP, a packet is referred to as a datagram. I’ll just use the term data packet here for the sake of simplicity.

Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) are two of the most well-known protocols in Layer 4.

TCP, a connection-oriented protocol, prioritizes data quality over speed.

TCP explicitly establishes a connection with the destination node and requires a handshake between the source and destination nodes when data is transmitted. The handshake confirms that data was received. If the destination node does not receive all of the data, TCP will ask for a retry.

TCP also ensures that packets are delivered or reassembled in the correct order. Learn more about TCP here .

UDP, a connectionless protocol, prioritizes speed over data quality. UDP does not require a handshake, which is why it’s called connectionless.

Because UDP doesn’t have to wait for this acknowledgement, it can send data at a faster rate, but not all of the data may be successfully transmitted and we’d never know.

If information is split up into multiple datagrams, unless those datagrams contain a sequence number, UDP does not ensure that packets are reassembled in the correct order. Learn more about UDP here .

TCP and UDP both send data to specific ports on a network device, which has an IP address. The combination of the IP address and the port number is called a socket.

Learn more about sockets here .

Learn more about the differences and similarities between these two protocols here .

How to Troubleshoot OSI Layer 4 Problems

Here are some Layer 4 problems to watch out for:

• Blocked ports - check your Access Control Lists (ACL) & firewalls
• Quality of Service (QoS) settings. QoS is a feature of routers/switches that can prioritize traffic, and they can really muck things up. Learn more about QoS here .

The Transport Layer provides end-to-end transmission of a message by segmenting a message into multiple data packets; the layer supports connection-oriented and connectionless communication.

OSI Layer 5

Layer 5 is the session layer . This layer establishes, maintains, and terminates sessions.

A session is a mutually agreed upon connection that is established between two network applications. Not two nodes! Nope, we’ve moved on from nodes. They were so Layer 4.

Just kidding, we still have nodes, but Layer 5 doesn’t need to retain the concept of a node because that’s been abstracted out (taken care of) by previous layers.

So a session is a connection that is established between two specific end-user applications. There are two important concepts to consider here:

• Client and server model: the application requesting the information is called the client, and the application that has the requested information is called the server.
• Request and response model: while a session is being established and during a session, there is a constant back-and-forth of requests for information and responses containing that information or “hey, I don’t have what you’re requesting.”

Sessions may be open for a very short amount of time or a long amount of time. They may fail sometimes, too.

Depending on the protocol in question, various failure resolution processes may kick in. Depending on the applications/protocols/hardware in use, sessions may support simplex, half-duplex, or full-duplex modes.

Examples of protocols on Layer 5 include Network Basic Input Output System (NetBIOS) and Remote Procedure Call Protocol (RPC), and many others.

From here on out (layer 5 and up), networks are focused on ways of making connections to end-user applications and displaying data to the user.

How to Troubleshoot OSI Layer 5 Problems

Here are some Layer 5 problems to watch out for:

• Servers are unavailable
• Servers are incorrectly configured, for example Apache or PHP configs
• Session failure - disconnect, timeout, and so on.

The Session Layer initiates, maintains, and terminates connections between two end-user applications. It responds to requests from the presentation layer and issues requests to the transport layer.

OSI Layer 6

Layer 6 is the presentation layer . This layer is responsible for data formatting, such as character encoding and conversions, and data encryption.

The operating system that hosts the end-user application is typically involved in Layer 6 processes. This functionality is not always implemented in a network protocol.

Layer 6 makes sure that end-user applications operating on Layer 7 can successfully consume data and, of course, eventually display it.

There are three data formatting methods to be aware of:

• American Standard Code for Information Interchange (ASCII): this 7-bit encoding technique is the most widely used standard for character encoding. One superset is ISO-8859-1, which provides most of the characters necessary for languages spoken in Western Europe.
• Extended Binary-Coded Decimal Interchange Code (EBDCIC): designed by IBM for mainframe usage. This encoding is incompatible with other character encoding methods.
• Unicode: character encodings can be done with 32-, 16-, or 8-bit characters and attempts to accommodate every known, written alphabet.

Learn more about character encoding methods in this article , and also here .

Encryption: SSL or TLS encryption protocols live on Layer 6. These encryption protocols help ensure that transmitted data is less vulnerable to malicious actors by providing authentication and data encryption for nodes operating on a network. TLS is the successor to SSL.

How to Troubleshoot OSI Layer 6 Problems

Here are some Layer 6 problems to watch out for:

• Non-existent or corrupted drivers
• Incorrect OS user access level

The Presentation Layer formats and encrypts data.

OSI Layer 7

Layer 7 is the application layer .

True to its name, this is the layer that is ultimately responsible for supporting services used by end-user applications. Applications include software programs that are installed on the operating system, like Internet browsers (for example, Firefox) or word processing programs (for example, Microsoft Word).

Applications can perform specialized network functions under the hood and require specialized services that fall under the umbrella of Layer 7.

Electronic mail programs, for example, are specifically created to run over a network and utilize networking functionality, such as email protocols, which fall under Layer 7.

Applications will also control end-user interaction, such as security checks (for example, MFA), identification of two participants, initiation of an exchange of information, and so on.

Protocols that operate on this level include File Transfer Protocol (FTP), Secure Shell (SSH), Simple Mail Transfer Protocol (SMTP), Internet Message Access Protocol (IMAP), Domain Name Service (DNS), and Hypertext Transfer Protocol (HTTP).

While each of these protocols serve different functions and operate differently, on a high level they all facilitate the communication of information. ( Source )

How to Troubleshoot OSI Layer 7 Problems

Here are some Layer 7 problems to watch out for:

• All issues on previous layers
• Incorrectly configured software applications
• User error (... we’ve all been there)

The Application Layer owns the services and functions that end-user applications need to work. It does not include the applications themselves.

Our Layer 1 koala is all grown up.

Learning check - can you apply makeup to a koala?

Don’t have a koala?

Well - answer these questions instead. It’s the next best thing, I promise.

• What is the OSI model?
• What are each of the layers?
• How could I use this information to troubleshoot networking issues?

Congratulations - you’ve taken one step farther to understanding the glorious entity we call the Internet.

Learning Resources

Many, very smart people have written entire books about the OSI model or entire books about specific layers. I encourage readers to check out any O’Reilly-published books about the subject or about network engineering in general.

Here are some resources I used when writing this article:

• The Illustrated Network, 2nd Edition
• Protocol Data Unit (PDU): https://www.geeksforgeeks.org/difference-between-segments-packets-and-frames/
• Troubleshooting Along the OSI Model: https://www.pearsonitcertification.com/articles/article.aspx?p=1730891
• The OSI Model Demystified: https://www.youtube.com/watch?v=HEEnLZV2wGI
• OSI Model for Dummies: https://www.dummies.com/programming/networking/layers-in-the-osi-model-of-a-computer-network/

Chloe Tucker is an artist and computer science enthusiast based in Portland, Oregon. As a former educator, she's continuously searching for the intersection of learning and teaching, or technology and art. Reach out to her on Twitter @_chloetucker and check out her website at chloe.dev .

Read more posts .

If you read this far, thank the author to show them you care. Say Thanks

Learn to code for free. freeCodeCamp's open source curriculum has helped more than 40,000 people get jobs as developers. Get started

  Layer 6 Presentation Layer

De/Encryption, Encoding, String representation

The presentation layer (data presentation layer, data provision level) sets the system-dependent representation of the data (for example, ASCII, EBCDIC) into an independent form, enabling the syntactically correct data exchange between different systems. Also, functions such as data compression and encryption are guaranteed that data to be sent by the application layer of a system that can be read by the application layer of another system to the layer 6. The presentation layer. If necessary, the presentation layer acts as a translator between different data formats, by making an understandable for both systems data format, the ASN.1 (Abstract Syntax Notation One) used.

OSI Layer 6 - Presentation Layer

The presentation layer is responsible for the delivery and formatting of information to the application layer for further processing or display. It relieves the application layer of concern regarding syntactical differences in data representation within the end-user systems. An example of a presentation service would be the conversion of an EBCDIC-coded text computer file to an ASCII-coded file. The presentation layer is the lowest layer at which application programmers consider data structure and presentation, instead of simply sending data in the form of datagrams or packets between hosts. This layer deals with issues of string representation - whether they use the Pascal method (an integer length field followed by the specified amount of bytes) or the C/C++ method (null-terminated strings, e.g. "thisisastring\0"). The idea is that the application layer should be able to point at the data to be moved, and the presentation layer will deal with the rest. Serialization of complex data structures into flat byte-strings (using mechanisms such as TLV or XML) can be thought of as the key functionality of the presentation layer. Encryption is typically done at this level too, although it can be done on the application, session, transport, or network layers, each having its own advantages and disadvantages. Decryption is also handled at the presentation layer. For example, when logging on to bank account sites the presentation layer will decrypt the data as it is received.[1] Another example is representing structure, which is normally standardized at this level, often by using XML. As well as simple pieces of data, like strings, more complicated things are standardized in this layer. Two common examples are 'objects' in object-oriented programming, and the exact way that streaming video is transmitted. In many widely used applications and protocols, no distinction is made between the presentation and application layers. For example, HyperText Transfer Protocol (HTTP), generally regarded as an application-layer protocol, has presentation-layer aspects such as the ability to identify character encoding for proper conversion, which is then done in the application layer. Within the service layering semantics of the OSI network architecture, the presentation layer responds to service requests from the application layer and issues service requests to the session layer. In the OSI model: the presentation layer ensures the information that the application layer of one system sends out is readable by the application layer of another system. For example, a PC program communicates with another computer, one using extended binary coded decimal interchange code (EBCDIC) and the other using ASCII to represent the same characters. If necessary, the presentation layer might be able to translate between multiple data formats by using a common format. Wikipedia

• Data conversion
• Character code translation
• Compression
• Encryption and Decryption

The Presentation OSI Layer is usually composed of 2 sublayers that are:

CASE common application service element

Sase specific application service element, layer 7   application layer, layer 6   presentation layer, layer 5   session layer, layer 4   transport layer, layer 3   network layer, layer 2   data link layer, layer 1   physical layer.

The OSI Model’s 7 Layers, Explained

The seven layers in the Open Systems Interconnection (OSI) model each serve a specific function and work together to create an efficient network communication system.

The Open Systems Interconnection (OSI) model is a framework in network communication that simplifies complex network interactions into a structured format. 

What Is the OSI Model?

The Open Systems Interconnection model is a framework in network communication designed to simplify complex network interactions into a structured format. This architecture has seven layers, each of which serves a specific function. All seven layers work together to create a robust and efficient network communication system.

Each of its seven layers has a distinct role, ensuring efficient data transfer from one device to another . The OSI model is essential for understanding how data is transmitted in a network and is also a practical guide for network protocol design and problem solving.

learn more about cybersecurity An Introduction to Microsegmentation in Network Security

The OSI model, developed by the International Organization for Standardization , outlines the essential functions of networking and telecommunications systems for practical application. It plays a crucial role in telecommunications, where vendors use it to define the features and capabilities of their products and services.

This approach allows for a detailed explanation of different aspects of network communication, including transport protocols, addressing schemes and data packaging methods. As a result, the OSI model resolves the complexities of network communication and fosters a more integrated and coherent digital world .

The 7 Layers of the OSI Model

Each layer of the OSI model serves a specific function, yet they work in harmony to create a robust and efficient network communication system. Understanding these layers provides valuable insights into the complexities of network design and operation, showcasing the intricate nature of modern digital communication.  

Layer 7: Application Layer

Functionality: The Application Layer is the closest to the end user. It facilitates user interaction with networked systems, providing interfaces and protocols for web browsers, email clients and other applications.

Key protocols: Protocols like HTTP, FTP and SMTP operate at this layer, enabling services such as web browsing, file transfers and email communications.

Layer 6: Presentation Layer

Role: The Presentation Layer acts as a translator, converting data formats from the application layer into a network-compatible format and vice versa. It ensures that data sent from one system is readable by another.

Data formatting: This layer is responsible for data encryption and compression, playing a significant role in maintaining data privacy and efficient transmission.

Layer 5: Session Layer

Managing sessions: It establishes, manages and terminates sessions between applications. This layer ensures that sessions are maintained for the duration of the communication.

Coordination: The Session Layer coordinates communication between systems, managing dialogues and synchronizing data exchange.

Layer 4: Transport Layer

Data segmentation and control: The Transport Layer is crucial for segmenting data into smaller packets. It ensures end-to-end data integrity and delivery, managing flow control, error correction and sequencing.

Protocols: TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) are key protocols in this layer, differing in their approach to data transmission.

Layer 3: Network Layer

Routing and addressing: This layer is responsible for logical addressing and routing data packets across different networks. It determines the best path for data to travel from source to destination.

Internet protocol: The Internet Protocol (IP), fundamental for internet data exchange, operates at this layer.

Layer 2: Data Link Layer

Framing and MAC addressing: The Data Link Layer frames data into packets. It handles physical addressing through MAC addresses, ensuring that data is directed to the correct hardware.

Error detection: This layer is also involved in error detection and handling, improving overall data transmission reliability.

Layer 1: Physical Layer

Physical transmission: The Physical Layer deals with the physical aspects of data transmission, including cable types, electrical signals and data rates.

Hardware components: It involves hardware components like cables, switches and network interface cards, forming the foundation of network communication.

How Data Flows in the OSI Model

Understanding this data flow process is crucial for professionals, as it aids in diagnosing and troubleshooting network issues, designing efficient network solutions and ensuring robust data security and management.

Encapsulation Process

When data is sent, it begins at the Application Layer and moves down through the layers. At each stage, it is encapsulated with the necessary headers, trailers, and other control information relevant to that layer. For instance, at the Transport Layer, data is segmented and encapsulated with port numbers, while at the Network Layer, IP addresses are added.

Each layer plays a role in preparing the data for transmission. The Presentation Layer may encrypt the data for security, while the Data Link Layer ensures it is formatted into frames suitable for physical transmission.

Data Transmission Across the Network

The Physical Layer transmits the raw bits over a physical medium, such as a cable or wireless network. This transmission is the actual movement of data across the network. In cases where data must move across different networks, the Network Layer’s routing functionalities become crucial. It ensures that data packets find the most efficient path to their destination.

Decapsulation Process

Upon reaching the destination, the data moves up the OSI model, with each layer removing its respective encapsulation. The Data Link Layer, for instance, removes framing, and the Transport Layer checks for transmission errors and reassembles the data segments. Once the data reaches the Application Layer, it is in its original format and ready to be used by the receiving application, whether it’s an email client, a web browser or any other networked software.

Seamless Data Flow

The OSI model ensures that each layer only communicates with its immediate upper and lower layers, creating a seamless flow. This layered approach means changes in one layer’s protocols or functionalities can occur without disrupting the entire network.

OSI Model Advantages

The OSI model is a cornerstone in network architecture for several reasons:

Simplification of network design

The OSI model’s layered approach breaks down complex network processes, making design and operation more manageable. Each layer focuses on a specific aspect of communication, allowing for independent development and easier troubleshooting.

Standardization and interoperability

It establishes universal standards for network communication, enabling different technologies to interact seamlessly. This interoperability is crucial for the efficient functioning of diverse network devices and applications.

Flexibility and Scalability

Adaptable to technological advancements, the OSI model allows individual layers to evolve without overhauling the entire system. This scalability makes it suitable for various network sizes and types.

Enhanced Security

Security measures are integrated at multiple layers, providing a robust defense against threats. Each layer can address specific security concerns, leading to comprehensive network protection.

Real-World Applications of the OSI Model

The OSI model’s influence extends well beyond theoretical concepts, playing a crucial role in various practical aspects of networking:

Network Design and Protocol Development

Network professionals use the OSI model as a blueprint for structuring and developing robust networks. It guides the creation of new protocols, ensuring seamless integration and functionality across different network layers.

Efficient Troubleshooting and Management

In troubleshooting, the OSI model provides a systematic approach for identifying issues, from physical connectivity to application-level errors. It also aids in network maintenance and performance optimization, addressing each layer to enhance overall efficiency.

Cybersecurity Strategy

The model is foundational in crafting layered security strategies . By implementing security measures at different layers, it offers comprehensive protection against various cyber threats. Understanding the OSI layers is key in detecting and mitigating attacks targeting specific network segments.

Educational and Training Tool

It serves as an essential framework in networking education, helping students and professionals alike understand complex network operations. The OSI model is a cornerstone in training programs , emphasizing the intricacies of network architecture and security.

safety first When and How to Run a Phishing Simulation

OSI Model vs. TCP/IP Model

While the OSI model offers a detailed conceptual framework, the TCP/IP model is recognized for its practical application in today’s internet-driven world.

Structural Differences

OSI model : Introduced as a comprehensive, protocol-independent framework, the OSI model details seven distinct layers, offering a more granular approach to network communication.

TCP/IP model : Developed earlier by the U.S. Department of Defense, the TCP/IP model consists of four layers (Application, Transport, Internet and Network Access), combining certain OSI layers.

Theoretical vs. Practical Approach

OSI model : Developed as a theoretical and universal networking model, it’s used more for educational purposes to explain how networks operate.

TCP/IP model : This model is designed around specific standard protocols, focusing on solving practical communication issues. It leaves sequencing and acknowledgment functions to the transport layer, differing from the OSI approach.

Adoption and Use

OSI model: While not widely implemented in its entirety, the OSI model’s clear layer separation is influential in protocol design and network education; simpler applications in the OSI framework may not utilize all seven layers, with only the first three layers (Physical, Data Link, and Network) being mandatory for basic data communication.

TCP/IP model : The dominant model used in most network architectures today, especially in internet-related communications. In TCP/IP, most applications engage all layers for communication.

Frequently Asked Questions

Why is the osi model important.

The OSI model is crucial for standardizing network communication and ensuring interoperability between various devices and systems. It simplifies network design and troubleshooting and serves as a fundamental educational tool in networking.

What are the 7 layers of the OSI model?

Layer 1: Physical Layer — Transmits raw data.

Layer 2: Data Link Layer — Manages direct links and framing.

Layer 3: Network Layer — Handles addressing and routing.

Layer 4: Transport Layer — Ensures reliable data transfer.

Layer 5: Session Layer — Manages connections.

Layer 6: Presentation Layer — Translates data formats.

Layer 7: Application Layer — Interfaces with applications.

Recent Expert Contributors Articles

• Artificial Intelligence
• Generative AI
• Cloud Computing
• CPUs and Processors
• Data Center
• Edge Computing
• Enterprise Storage
• Virtualization
• Internet of Things
• Network Management Software
• Network Security
• Enterprise Buyer’s Guides
• United States
• Newsletters
• Foundry Careers
• Terms of Service
• Privacy Policy
• Cookie Policy
• Copyright Notice
• Member Preferences
• About AdChoices
• E-commerce Links
• Your California Privacy Rights

Our Network

• Computerworld

The OSI model explained and how to easily remember its 7 layers

A tutorial on the open systems interconnection networking reference model and tips on and how to memorize the seven layers.

The Open Systems Interconnect (OSI) model is a conceptual framework that describes networking or telecommunications systems as seven layers, each with its own function.

The layers help network pros visualize what is going on within their networks and can help network managers narrow down problems (is it a physical issue or something with the application?), as well as computer programmers (when developing an application, which other layers does it need to work with?). Tech vendors selling new products will often refer to the OSI model to help customers understand which layer their products work with or whether it works “across the stack”.

The 7 layers of the OSI model

The layers are: Layer 1—Physical; Layer 2—Data Link; Layer 3—Network; Layer 4—Transport; Layer 5—Session; Layer 6—Presentation; Layer 7—Application.

It wasn’t always this way. Conceived in the 1970s when computer networking was taking off, two separate models were merged in 1983 and published in 1984 to create the OSI model that most people are familiar with today. Most descriptions of the OSI model go from top to bottom, with the numbers going from Layer 7 down to Layer 1. The layers, and what they represent, are as follows:

Layer 7 – Application

The Application Layer in the OSI model is the layer that is the “closest to the end user”. It receives information directly from users and displays incoming data to the user. Oddly enough, applications themselves do not reside at the application layer. Instead the layer facilitates communication through lower layers in order to establish connections with applications at the other end. Web browsers (Google Chrome, Firefox, Safari, etc.) TelNet, and FTP, are examples of communications  that rely  on Layer 7.

Layer 6 – Presentation

The Presentation Layer represents the area that is independent of data representation at the application layer. In general, it represents the preparation or translation of application format to network format, or from network formatting to application format. In other words, the layer “presents” data for the application or the network. A good example of this is encryption and decryption of data for secure transmission; this happens at Layer 6.

Layer 5 – Session

When two computers or other networked devices need to speak with one another, a session needs to be created, and this is done at the Session Layer . Functions at this layer involve setup, coordination (how long should a system wait for a response, for example) and termination between the applications at each end of the session.

Layer 4 – Transport

The Transport Layer deals with the coordination of the data transfer between end systems and hosts. How much data to send, at what rate, where it goes, etc. The best known example of the Transport Layer is the Transmission Control Protocol (TCP), which is built on top of the Internet Protocol (IP), commonly known as TCP/IP. TCP and UDP port numbers work at Layer 4, while IP addresses work at Layer 3, the Network Layer.

Layer 3 – Network

Here at the Network Layer is where you’ll find most of the router functionality that most networking professionals care about and love. In its most basic sense, this layer is responsible for packet forwarding, including routing through different routers . You might know that your Boston computer wants to connect to a server in California, but there are millions of different paths to take. Routers at this layer help do this efficiently.

Layer 2 – Data Link

The Data Link Layer provides node-to-node data transfer (between two directly connected nodes), and also handles error correction from the physical layer. Two sublayers exist here as well–the Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. In the networking world, most switches operate at Layer 2. But it’s not that simple. Some switches also operate at Layer 3 in order to support virtual LANs that may span more than one switch subnet, which requires routing capabilities.

Layer 1 – Physical

At the bottom of our OSI model we have the Physical Layer, which represents the electrical and physical representation of the system. This can include everything from the cable type, radio frequency link (as in a Wi-Fi network), as well as the layout of pins, voltages, and other physical requirements. When a networking problem occurs, many networking pros go right to the physical layer to check that all of the cables are properly connected and that the power plug hasn’t been pulled from the router, switch or computer, for example.

Why you need to know the 7 OSI layers

Most people in IT will likely need to know about the different layers when they’re going for their certifications, much like a civics student needs to learn about the three branches of the US government. After that, you hear about the OSI model when vendors are making pitches about which layers their products work with.

In a Quora post  asking about the purpose of the OSI model, Vikram Kumar answered this way:

“The purpose of the OSI reference model is to guide vendors and developers so the digital communication products and software programs they create will interoperate, and to facilitate clear comparisons among communications tools.”

While some people may argue that the OSI model is obsolete (due to its conceptual nature) and less important than the four layers of the TCP/IP model, Kumar says that “it is difficult to read about networking technology today without seeing references to the OSI model and its layers, because the model’s structure helps to frame discussions of protocols and contrast various technologies.”

If you can understand the OSI model and its layers, you can also then understand which protocols and devices can interoperate with each other when new technologies are developed and explained.

The OSI model remains relevant

In a post on GeeksforGeeks, contributor Vabhav Bilotia argues several reasons why the OSI model remains relevant, especially when it comes to security and determining where technical risks and vulnerabilities may exist.

For example, by understanding the different layers, enterprise security teams can identify and classify physical access, where the data is sitting, and provide an inventory of the applications that employees use to access data and resources.

“Knowing where the majority of your company’s data is held, whether on-premises or in cloud services, will help define your information security policy,” writes Bilotia. “You can invest in the correct solutions that provide you data visibility within the proper OSI layers once you have this knowledge.”

In addition, the OSI model can be used to understand cloud infrastructure migrations, particularly when it comes to securing data within the cloud.

And because the model has been around for so long and understood by so many, the uniform vocabulary and terms helps networking professionals understand quickly about the components of the networking system “While this paradigm is not directly implemented in today’s TCP/IP networks, it is a useful conceptual model for relating multiple technologies to one another and implementing the appropriate technology in the appropriate way,” Bilotia writes. We couldn’t agree more.

How to remember the OSI Model 7 layers – 8 mnemonic tricks

If you need to memorize the layers for a college or certification test, here are a few sentences to help remember them in order. The first letter of each word is the same as the first letter an OSI layer.

From Application to Physical (Layer 7 to Layer 1): 

• All People Seem To Need Data Processing
• All Pros Search Top Notch Donut Places
• A Penguin Said That Nobody Drinks Pepsi
• A Priest Saw Two Nuns Doing Pushups

From Physical to Application (Layer 1 to Layer 7):

• Please Do Not Throw Sausage Pizza Away
• Pew! Dead Ninja Turtles Smell Particularly Awful
• People Don’t Need To See Paula Abdul
• Pete Doesn’t Need To Sell Pickles Anymore

Keith Shaw was a Network World editor and the writer of the Cool Tools column. He is now a freelance writer and editor from Worcester, Mass.

Related content

Hpe aruba looks to fight ai threats with ai weapons, ai features boost cisco's panoptica application security software, red hat extends lightspeed generative ai tool to openshift and enterprise linux, red hat introduces 'policy as code' for ansible, newsletter promo module test.

The first gadget Keith Shaw ever wanted was the Merlin, a red plastic toy that beeped and played Tic-Tac-Toe and various other games. A child of the '70s and teenager of the '80s, Shaw has been a fan of computers, technology and video games right from the start. He won an award in 8th grade for programming a game on the school's only computer, and saved his allowance to buy an Atari 2600.

Shaw has a bachelor's degree in newspaper journalism from Syracuse University and has worked at a variety of newspapers in New York, Florida and Massachusetts, as well as Computerworld and Network World. He won an award from the American Society of Business Publication Editors for a 2003 article on anti-spam testing, and a Gold Award in their 2010 Digital Awards Competition for the "ABCs of IT" video series.

Shaw is also the co-creator of taquitos.net , the crunchiest site on the InterWeb, which has taste-tested and reviewed more than 4,000 varieties of snack foods.

More from this author

What is a virtual machine, and why are they so useful, what is a network switch and how does it work, what is zero trust network access, what is sd-wan, and what does it mean for networking, security, cloud, most popular authors.

• Elizabeth Montalbano

Show me more

Riverbed launches ai-powered observability platform.

Tata Communications launches edge computing platform for enterprises

HPE launches storage system for HPC and AI clusters

Has the hype around ‘Internet of Things’ paid off? | Ep. 145

Episode 1: Understanding Cisco’s Converged SDN Transport

Episode 2: Pluggable Optics and the Internet for the Future

Has the hype around ‘Internet of Things’ paid off?

Are unused IPv4 addresses a secret gold mine?

Preparing for a 6G wireless world: Exciting changes coming to the wireless industry

How-To Geek

The 7 osi networking layers explained.

The Open Systems Interconnection (OSI) networking model defines a conceptual framework for communications between computer systems.

Quick Links

• Physical Layer
• Data Link Layer
• Network Layer
• Transport Layer
• Session Layer
• Presentation Layer
• Application Layer

The Open Systems Interconnection (OSI) networking model defines a conceptual framework for communications between computer systems. The model is an ISO standard which identifies seven fundamental networking layers, from the physical hardware up to high-level software applications.

Each layer in the model handles a specific networking function. The standard helps administrators to visualize networks, isolate problems, and understand the use cases for new technologies. Many network equipment vendors advertise the OSI layer that their products are designed to slot into.

OSI was adopted as an international standard in 1984. It remains relevant today despite the changes to network implementation that have occurred since first publication. Cloud, edge, and IoT can all be accommodated within the model.

In this article, we'll explain each of the seven OSI layers in turn. We'll start from the lowest level, labelled as Layer 1.

1. Physical Layer

All networking begins with physical equipment. This layer encapsulates the hardware involved in the communications, such as switches and cables. Data is transferred as a stream of binary digits - 0 or 1 - that the hardware prepares from input it's been fed. The physical layer specifies the electrical signals that are used to encode the data over the wire, such as a 5-volt pulse to indicate a binary "1."

Errors in the physical layer tend to result in data not being transferred at all. There could be a break in the connection due to a missing plug or incorrect power supply. Problems can also arise when two components disagree on the physical encoding of data values. In the case of wireless connections, a weak signal can lead to bit loss during transmission.

2. Data Link Layer

The model's second layer concerns communication between two devices that are directly connected to each other in the same network. It's responsible for establishing a link that allows data to be exchanged using an agreed protocol. Many network switches operate at Layer 2.

The data link layer will eventually pass bits to the physical layer. As it sits above the hardware, the data link layer can perform basic error detection and correction in response to physical transfer issues. There are two sub-layers that define these responsibilities: Logical Link Control (LLC) that handles frame synchronization and error detection, and Media Access Control (MAC) which uses MAC addresses to constrain how devices acquire permission to transfer data.

3. Network Layer

The network layer is the first level to support data transfer between two separately maintained networks. It's redundant in situations where all your devices exist on the same network.

Data that comes to the network layer from higher levels is first broken up into packets suitable for transmission. Packets received from the remote network in response are reassembled into usable data.

The network layer is where several important protocols are first encountered. These include IP (for determining the path to a destination), ICMP, routing, and virtual LAN. Together these mechanisms facilitate inter-network communications with a familiar degree of usability. However operations at this level aren't necessarily reliable: messages aren't required to succeed and may not necessarily be retried.

4. Transport Layer

The transport layer provides higher-level abstractions for coordinating data transfers between devices. Transport controllers determine where data will be sent and the rate it should be transferred at.

Layer 4 is where TCP and UDP are implemented, providing the port numbers that allow devices to expose multiple communication channels. Load balancing is often situated at Layer 4 as a result, allowing traffic to be routed between ports on a target device.

Transport mechanisms are expected to guarantee successful communication. Stringent error controls are applied to recover from packet loss and retry failed transfers. Flow control is enforced so the sender doesn't overwhelm the remote device by sending data more quickly than the available bandwidth permits.

5. Session Layer

Layer 5 creates ongoing communication sessions between two devices. Sessions are used to negotiate new connections, agree on their duration, and gracefully close down the connection once the data exchange is complete. This layer ensures that sessions remain open long enough to transfer all the data that's being sent.

Checkpoint control is another responsibility that's held by Layer 5. Sessions can define checkpoints to facilitate progress updates and resumable transmissions. A new checkpoint could be set every few megabytes for a file upload, allowing the sender to continue from a particular point if the transfer gets interrupted.

Many significant protocols operate at Layer 5 including authentication and logon technologies such as LDAP and NetBIOS. These establish semi-permanent communication channels for managing an end user session on a specific device.

6. Presentation Layer

The presentation layer handles preparation of data for the application layer that comes next in the model. After data has made it up from the hardware, through the data link, and across the transport, it's almost ready to be consumed by high-level components. The presentation layer completes the process by performing any formatting tasks that may be required.

Decryption, decoding, and decompression are three common operations found at this level. The presentation layer processes received data into formats that can be eventually utilized by a client application. Similarly, outward-bound data is reformatted into compressed and encrypted structures that are suitable for network transmission.

TLS is one major technology that's part of the presentation layer. Certificate verification and data decryption is handled before requests reach the network client, allowing information to be consumed with confidence that it's authentic.

7. Application Layer

The application layer is the top of the stack. It represents the functionality that's perceived by network end users. Applications in the OSI model provide a convenient end-to-end interface to facilitate complete data transfers, without making you think about hardware, data links, sessions, and compression.

Despite its name, this layer doesn't relate to client-side software such as your web browser or email client. An application in OSI terms is a protocol that caters for the complete communication of complex data through layers 1-6.

HTTP, FTP, DHCP, DNS, and SSH all exist at the application layer. These are high-level mechanisms which permit direct transfers of user data between an origin device and a remote server. You only need minimal knowledge of the workings of the other layers.

The seven OSI layers describe the transfer of data through computer networks. Understanding the functions and responsibilities of each layer can help you identify the source of problems and assess the intended use case for new components.

OSI is an abstract model that doesn't directly map to the specific networking implementations commonly used today. As an example, the TCP/IP protocol works on its own simpler system of four layers: Network Access, Internet, Transport, and Application. These abstract and absorb the equivalent OSI layers: the application layer spans OSI L5 to L7, while L1 and L2 are combined in TCP/IP's concept of Network Access.

OSI remains applicable despite its lack of direct real-world application. It's been around so long that it's widely understood among administrators from all backgrounds. Its relatively high level of abstraction has also ensured it's remained relevant in the face of new networking paradigms, many of which have targeted Layer 3 and above. An awareness of the seven layers and their responsibilities can still help you appreciate the flow of data through a network while uncovering integration opportunities for new components.

• Editor's Choice: Tech Gifts for Mom
• New! iPad Air 6 Details

The Layers of the OSI Model Illustrated

Each layer explained

• Massachusetts Institute of Technology
• University of Illinois

• The Wireless Connection
• Routers & Firewalls
• Network Hubs
• Installing & Upgrading
• Wi-Fi & Wireless

The Open Systems Interconnection (OSI) model defines a networking framework to implement protocols in layers, with control passed from one layer to the next. It is primarily used today as a teaching tool. It conceptually divides computer network architecture into 7 layers in a logical progression.

The lower layers deal with electrical signals, chunks of binary data , and routing of these data across networks. Higher levels cover network requests and responses, representation of data, and network protocols, as seen from a user's point of view. 

The OSI model was originally conceived as a standard architecture for building network systems, and many popular network technologies today reflect the layered design of OSI.

Physical Layer

At Layer 1, the Physical layer of the OSI model is responsible for the ultimate transmission of digital data bits from the Physical layer of the sending (source) device over network communications media to the Physical layer of the receiving (destination) device.

Examples of layer 1 technologies include  Ethernet cables  and  hubs . Also, hubs and other repeaters  are standard network devices that function at the Physical layer, as are cable connectors.

At the Physical layer, data is transmitted using the type of signaling supported by the physical medium: electric voltages, radio frequencies, or pulses of infrared or ordinary light.

Data Link Layer

When obtaining data from the Physical layer, the Data Link layer checks for physical transmission errors and packages bits into data frames. The Data Link layer also manages physical addressing schemes such as MAC addresses for Ethernet networks, controlling access of network devices to the physical medium.

Because the Data Link layer is the most complex layer in the OSI model, it is often divided into two parts: the Media Access Control sub-layer and the Logical Link Control sub-layer.

Network Layer

The Network layer adds the concept of routing above the Data Link layer. When data arrives at the Network layer, the source and destination addresses contained inside each frame are examined to determine if the data has reached its final destination. If the data has reached the final destination, layer 3 formats the data into packets delivered to the Transport layer. Otherwise, the Network layer updates the destination address and pushes the frame down to the lower layers.

To support routing, the Network layer maintains logical addresses such as IP addresses  for devices on the network. The Network layer also manages the mapping between these logical addresses and physical addresses. In IPv4 networking, this mapping is accomplished through the Address Resolution Protocol (ARP); IPv6 uses Neighbor Discovery Protocol (NDP).

Transport Layer

The Transport Layer delivers data across network connections. TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) are the most common examples of Transport Layer 4 network protocols.  Different transport protocols may support a range of optional capabilities, including error recovery, flow control, and support for re-transmission.

Session Layer

The Session Layer manages the sequence and flow of events that initiate and tear down network connections. At layer 5, it is built to support multiple types of connections that can be created dynamically and run over individual networks.

Presentation Layer

The Presentation layer has the simplest function of any piece of the OSI model. At layer 6, it handles syntax processing of message data such as format conversions and encryption/decryption needed to support the Application layer above it.

Application Layer

The Application layer supplies network services to end-user applications. Network services are protocols that work with the user's data. For example, in a web browser application, the Application layer protocol HTTP packages the data needed to send and receive web page content. This layer 7 provides data to (and obtains data from) the Presentation layer.

Get the Latest Tech News Delivered Every Day

• Common Questions and Answers on the OSI Network Model
• A Guide to X.25 in Computer Networking
• OSI Model Reference Guide
• Wireless Access Point vs. Wireless Application Protocol
• Visual Index of Computer Networking Topics
• Here's Why Your Network Might Need a Layer 3 Switch
• Understanding Transmission Control Protocol/Internet Protocol (TCP/IP)
• What Is PPTP: Point-to-Point Tunneling Protocol
• How Do Computer Network Protocols Work?
• Beginner's Guide to Asynchronous Transfer Mode (ATM)
• ARP (Address Resolution Protocol) and Your Computer Network
• What Is a Virtual LAN (VLAN)?
• Use a Bridge to Expand Your Local Network
• A Short Guide on Networking Fundamentals
• What Are Network Protocols?
• What Is a Switch?

Objective 4.1: The OSI Model

Objective 4.1: Explain the function of each layer of the OSI (Open Systems Interconnection) model

The Open Systems Interconnection Reference Model (OSI Reference Model or OSI Model) is an abstract description for layered communications and computer network protocol design. In its most basic form, it divides network architecture into seven layers which, from top to bottom, are the Application, Presentation, Session, Transport, Network, Data Link, and Physical Layers. It is therefore often referred to as the OSI Seven Layer Model.

A layer is a collection of conceptually similar functions that provide services to the layer above it and receives service from the layer below it. For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of the path.

Reference Appendix A: Memory Aids to assist in the recall of the layers in the correct order.

Application [ edit | edit source ]

The application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network resources for the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer.

Presentation [ edit | edit source ]

The presentation layer establishes context between application-layer entities, in which the higher-layer entities may use different syntax and semantics if the presentation service provides a mapping between them. If a mapping is available, presentation service data units are encapsulated into session protocol data units, and passed down the stack.

This layer provides independence from data representation (e.g., encryption) by translating between application and network formats. The presentation layer transforms data into the form that the application accepts. This layer formats and encrypts data to be sent across a network. It is sometimes called the syntax layer.[5]

The original presentation structure used the basic encoding rules of Abstract Syntax Notation One (ASN.1), with capabilities such as converting an EBCDIC-coded text file to an ASCII-coded file, or serialization of objects and other data structures from and to XML.

Session [ edit | edit source ]

The session layer controls the dialogues (connections) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for graceful close of sessions, which is a property of the Transmission Control Protocol, and also for session checkpointing and recovery, which is not usually used in the Internet Protocol Suite. The session layer is commonly implemented explicitly in application environments that use remote procedure calls.

Transport [ edit | edit source ]

The transport layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The transport layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state- and connection-oriented. This means that the transport layer can keep track of the segments and retransmit those that fail. The transport layer also provides the acknowledgement of the successful data transmission and sends the next data if no errors occurred.

OSI defines five classes of connection-mode transport protocols ranging from class 0 (which is also known as TP0 and provides the least features) to class 4 (TP4, designed for less reliable networks, similar to the Internet). Class 0 contains no error recovery, and was designed for use on network layers that provide error-free connections. Class 4 is closest to TCP, although TCP contains functions, such as the graceful close, which OSI assigns to the session layer. Also, all OSI TP connection-mode protocol classes provide expedited data and preservation of record boundaries. Detailed characteristics of TP0-4 classes are shown in the following table: [1]

Perhaps an easy way to visualize the transport layer is to compare it with a Post Office, which deals with the dispatch and classification of mail and parcels sent. Do remember, however, that a post office manages the outer envelope of mail. Higher layers may have the equivalent of double envelopes, such as cryptographic presentation services that can be read by the addressee only. Roughly speaking, tunneling protocols operate at the transport layer, such as carrying non-IP protocols such as IBM's IBM Systems Network Architecture|SNA or Novell's Internetwork Packet Exchange|IPX over an IP network, or end-to-end encryption with IPsec. While Generic Routing Encapsulation (GRE) might seem to be a network-layer protocol, if the encapsulation of the payload takes place only at endpoint, GRE becomes closer to a transport protocol that uses IP headers but contains complete frames or packets to deliver to an endpoint. layer 2 Tunneling Protocol|L2TP carries Point-to-Point Protocol|PPP frames inside transport packet.

Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of the transport layer, the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) of the Internet Protocol Suite are commonly categorized as layer-4 protocols within OSI.

Network [ edit | edit source ]

The network layer provides the functional and procedural means of transferring variable length data sequences from a source host on one network to a destination host on a different network, while maintaining the quality of service requested by the transport layer (in contrast to the data link layer which connects hosts within the same network). The network layer performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors. Routers operate at this layer, sending data throughout the extended network and making the Internet possible. This is a logical addressing scheme – values are chosen by the network engineer. The addressing scheme is not hierarchical.

The network layer may be divided into three sublayers:

• Subnetwork access – that considers protocols that deal with the interface to networks, such as X.25;
• Subnetwork-dependent convergence – when it is necessary to bring the level of a transit network up to the level of networks on either side
• Subnetwork-independent convergence – handles transfer across multiple networks.

An example of this latter case is CLNP, or IPv7 ISO 8473. It manages the connectionless transfer of data one hop at a time, from end system to ingress router, router to router, and from egress router to destination end system. It is not responsible for reliable delivery to a next hop, but only for the detection of erroneous packets so they may be discarded. In this scheme, IPv4 and IPv6 would have to be classed with X.25 as subnet access protocols because they carry interface addresses rather than node addresses.

A number of layer-management protocols, a function defined in the Management Annex, ISO 7498/4, belong to the network layer. These include routing protocols, multicast group management, network-layer information and error, and network-layer address assignment. It is the function of the payload that makes these belong to the network layer, not the protocol that carries them.

Data Link [ edit | edit source ]

The data link layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the physical layer. Originally, this layer was intended for point-to-point and point-to-multipoint media, characteristic of wide area media in the telephone system. Local area network architecture, which included broadcast-capable multiaccess media, was developed independently of the ISO work in IEEE Project 802. IEEE work assumed sublayering and management functions not required for WAN use. In modern practice, only error detection, not flow control using sliding window, is present in data link protocols such as Point-to-Point Protocol (PPP), and, on local area networks, the IEEE 802.2 LLC layer is not used for most protocols on the Ethernet, and on other local area networks, its flow control and acknowledgment mechanisms are rarely used. Sliding window flow control and acknowledgment is used at the transport layer by protocols such as TCP, but is still used in niches where X.25 offers performance advantages.

The ITU-T G.hn standard, which provides high-speed local area networking over existing wires (power lines, phone lines and coaxial cables), includes a complete data link layer which provides both error correction and flow control by means of a selective repeat Sliding Window Protocol.

Both WAN and LAN service arrange bits, from the physical layer, into logical sequences called frames. Not all physical layer bits necessarily go into frames, as some of these bits are purely intended for physical layer functions. For example, every fifth bit of the FDDI bit stream is not used by the layer.

Physical [ edit | edit source ]

The physical layer defines electrical and physical specifications for devices. In particular, it defines the relationship between a device and a transmission medium, such as a copper or optical cable. This includes the layout of pins, voltages, cable specifications, hubs, repeaters, network adapters, host bus adapters (HBA used in storage area networks) and more.

The major functions and services performed by the physical layer are:

• Establishment and termination of a connection to a communications medium.
• Participation in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control.
• Modulation, or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling (such as copper and optical fiber) or over a radio link.

Parallel SCSI buses operate in this layer, although it must be remembered that the logical SCSI protocol is a transport layer protocol that runs over this bus. Various physical-layer Ethernet standards are also in this layer; Ethernet incorporates both this layer and the data link layer. The same applies to other local-area networks, such as token ring, FDDI, ITU-T G.hn and IEEE 802.11, as well as personal area networks such as Bluetooth and IEEE 802.15.4.

References [ edit | edit source ]

• Book:Network Plus Certification

Navigation menu

Learn the OSI model in 5 minutes

Opensource.com

The Open Systems Interconnection (OSI) model is a standard for how computers, servers, and people communicate within a system. It was the first standard model for network communications and was adopted in the early 1980s by all major computer and telecommunications companies.

The OSI model provides a universal language for describing networks and thinking about them in discrete chunks, or layers.

Layers of the OSI model

The model describes the seven layers through which computer systems communicate over a network.

Application layer

Presentation layer, session layer, transport layer, network layer, data link layer, physical layer.

Each of these layers has its own way of working, with its own set of protocols that distinguish it from the others. This article provides a breakdown of the layers one by one.

The application layer is implemented in software. It is the layer used to interact with applications.

Consider the example of sending a message. The sender will interact with the application layer and send the message. The application layer sends the message to the next layer in the OSI Model, the presentation layer.

The data from the application layer is forwarded to the presentation layer. The presentation layer receives the data in the form of words, characters, letters, numbers, and so on, and converts them into machine representable binary format. This process is known as translation .

At this stage, ASCII characters (American Standard Code for Information Interchange) are converted into Extended Binary Coded Decimal Interchange Code (EBCDIC). Before the converted data goes further, it also undergoes encoding and encryption processes, using the SSL protocol for encryption and decryption.

The presentation layer provides abstraction and assumes that the layers following it will take care of the data forwarded to them from this layer. It also plays a role in compression of the data. The compression can be lossy or lossless, depending on various factors beyond this article's scope.

The session layer helps in setting up and managing connections. The main work of this layer is to establish a session. For example, on an online shopping site, a session is created between your computer and the site's server.

The session layer enables the sending and receiving of data, followed by the termination of connected sessions. Authentication is done before a session is established, followed by authorization . Like the previous layers, the session layer also assumes that, after its work is done, the data will be correctly handled by the subsequent layers.

The transport layer manages data transportation and its own set of protocols for how data will be transferred. The data received here from the session layer is divided into smaller data units called segments . This process is known as segmentation . Every segment contains the source's and destination's port numbers and a sequence number. Port numbers identify the application on which the data needs to be sent. Note that the data is transferred in chunks. The sequence numbers are used to reassemble the segments in the correct order.

The transport layer takes care of the flow control , or the amount of data transferred at a given time. It also accounts for error control , such as data loss, data corruption, and so on. It makes use of an error-detecting value known as a checksum . The transport layer adds a checksum to every data segment to check whether the sent data is received correctly. Data is then transferred to the network layer.

The network layer helps communicate with other networks. It works to transmit received data segments from one computer to another located in a different network. The router lives in the network layer.

The function of the network layer is logical addressing (IP Addressing). It assigns the sender's and receiver's IP addresses to each data packet to ensure it is received at the correct destination. The network layer then routes the data packets.  Load balancing  also happens in the network layer to make sure that no overloading takes place. Next, the data is transported to the data link layer.

The data link layer allows direct communication with other devices, such as computers and hosts.

It receives data packets containing the IP addresses of the sender and receiver from the network layer and does the physical addressing, assigning the media access control (MAC) addresses of the sender and receiver to a data packet to form a frame .

This layer consists of all the hardware and mechanical elements of a system, including the configuration of wires, pins, adapters, and so forth. The data received here by the preceding layers is in the form of 0s and 1s. The physical layer converts this data and transports it to local media via various means, including wires, electrical signals, light signals (as in optical fiber cables), and radio signals (as in WiFi).

Note that the physical layer works at the receiver's end and transports the received signal to the data link as a frame (by converting it back to bits). The frame is moved to the higher layers, and ultimately the required data is received at the application layer, which is the software.

The OSI model is helpful when you need to describe network architecture or troubleshoot network problems. I hope this article gave you a clearer understanding of the elements this model. 

How Linux came to the mainframe

Linux's emergence on the mainframe is indebted to the external developer community working with IBM.

Related Content

• A+ Cert Exam
• Network + Cert Exam
• Server+ Cert Exam
• Security+ Cert Exam
• CCNA Cert Exam
• CCNP Cert Exam
• ITIL Foundation Cert Exam
• PMP Cert Exam
• JNCIA-Junos Cert Exam
• OCPJP Cert Exam
• OCA Cert Exam
• CIW Associate Cert Exam
• SMT Component & Assembly
• Reliability Enhancement
• SMD Components
• Nano Technology Introduction
• Fundamentals of SatCom
• Link Budgeting Explained
• Satellite link budget calculator
• Satellite look-angle calculator
• Satellite Phones
• RF, Microwave, and Optical Comm. - A review
• Satellites and Services
• Motherboard Connectors On a PC
• Motherboard-I
• Motherboard-II
• Flash Memory Tutorial
• Printers and Scanners
• Cellular Phones
• OSI Model: A tutorial
• Introduction to TCP/IP

Computer Networking

• Routers-IOS & JUNOS
• Wimax,LTE, and 3G/3.5G - Intro
• WiMAX: Wireless Broadband
• Broadband Internet Technologies
• RFID: A Beginner's Guide
• Spread Spectrum Comms.
• A tutorial on IPv6
• Cloud Computing
• Introduction to MPLS
• Initial Software Config on SRX100
• comptia aplus core 1 study notes
• comptia aplus core 2 study notes
• comptia Networkplus study notes
• comptia Securityplus study notes
• comptia Serverplus study notes
• comptia IT Fundamentals study notes
• Cisco CCNA study notes
• Cisco CCNP ENCOR study notes
• Cisco CCNP ENARSI study notes
• Cisco CCST Networking study notes
• Cisco CCST Cybersecurity study notes
• Juniper JNCIA study notes
• Co-axial Cable Measurements
• Spectrum Analyzers Intro
• Noise Figure Measurement
• Time/Frequency Standards
• Coax Cables - Intro
• Android App Development
• Essentials of Webpage Development
• Introduction to HTML5
• Introduction to Java
• Introduction to MySQL
• ActiveX EXE:Beginner's Guide
• Working w/ MyODBC & MySQL
• Drag-n-Drop Using Visual Basic
• Drag-n-Drop Using VB.net
• Network Simulator-2 (NS2)
• Computers & Networking
• The OSI (Open Systems Interconnection) model

The OSI (Open Systems Interconnection) Model

1. the osi (open systems interconnection) model.

Definition: The OSI model defines internetworking in terms of a vertical stack of seven layers. The upper layers of the OSI model represent software that implements network services like encryption and connection management. The lower layers of the OSI model implement more primitive, hardware-oriented functions like routing, addressing, and flow control.

The OSI model was introduced in 1984. Although it was designed to be an abstract model, the OSI model remains a practical framework for today's key network technologies like Ethernet and protocols like IP.

The OSI model should be used as a guide for how data is transmitted over the network. It is an abstract representation of the data pathway and should be treated as such.

The OSI model was specifically made for connecting open systems. These systems are designed to be open for communication with almost any other system. The model was made to break down each functional layer so that overall design complexity could be lessened. The model was constructed with seven layers for the flow of information. These are:

• Application Layer
• Presentation layer
• Session layer
• Transport layer
• Network layer
• Data link layer
• Physical layer

1.1 Application layer

Provides a means for the user to access information on the network through an application. This layer is the main interface for the user to interact with the application and therefore the network.

The application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network resources for the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer.

Some examples of application layer implementations include Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).

1.2 Presentation layer

Manages the presentation of the information in an ordered and meaningful manner. This layer's primary function is the syntax and semantics of the data transmission. It converts local host computer data representations into a standard network format for transmission on the network. On the receiving side, it changes the network format into the appropriate host computer's format so that data can be utilized independent of the host computer. ASCII and EBCDIC conversions, cryptography, and the like are handled here.

The presentation layer provides a variety of coding and conversion functions that are applied to application layer data. These functions ensure that information sent from the application layer of one system would be readable by the application layer of another system. Some examples of presentation layer coding and conversion schemes include common data representation formats, conversion of character representation formats, common data compression schemes, and common data encryption schemes.

Common data representation formats, or the use of standard image, sound, and video formats, enable the interchange of application data between different types of computer systems. Using different text and data representations, such as EBCDIC and ASCII, uses conversion schemes to exchange information with systems. Standard data compression schemes enable data that is compressed. or encrypted at the source device to be properly decompressed, or deciphered at the destination.

Presentation layer implementations are not typically associated with a particular protocol stack. Some well-known standards for video include QuickTime and Motion Picture Experts Group (MPEG). QuickTime is an Apple Computer specification for video and audio, and MPEG is a standard for video compression and coding.

Among the well-known graphic image formats are Graphics Interchange Format (GIF), Joint Photographic Experts Group (JPEG), and Tagged Image File Format (TIFF). GIF is a standard for compressing and coding graphic images. JPEG is another compression and coding standard for graphic images, and TIFF is a standard coding format for graphic images.

1.3 Session layer

Coordinates dialogue/session/connection between devices over the network. This layer manages communications between connected sessions. Examples of this layer are token management (the session layer manages who has the token) and network time synchronization.

The session layer establishes, manages, and terminates communication sessions. Communication sessions consist of service requests and service responses that occur between applications located in different network devices. These requests and responses are coordinated by protocols implemented at the session layer. Some examples of session-layer implementations include Zone Information Protocol (ZIP), the AppleTalk protocol that coordinates the name binding process; and Session Control Protocol (SCP), the Decent Phase IV session layer protocol.

1.4 Transport layer

Responsible for reliable transmission of data and service specification between hosts. The major responsibility of this layer is data integrity--that data transmitted between hosts is reliable and timely. Upper layer data grams are broken down into network-sized data grams if needed and then implemented using appropriate transmission control. The transport layer creates one or more than one network connection, depending on conditions. This layer also handles what type of connection will be created. Two major transport protocols are the TCP (Transmission Control Protocol) and the UDP (User Data gram Protocol).

Important features of Transport layer:

• Transport layer ensures reliable service.
• Breaks the message (from sessions layer) into smaller packets, assigns sequence number and sends them.
• Reliable transport connections are built on top of X.25 or IP.
• In case IP, lost packets arriving out of order must be reordered.

Important features of TCP/UDP:

• TCP/IP Widely used for network/transport layer (UNIX).
• TCP (Transport Control Protocol): This is a connection oriented protocol.
• UDP (Universal Data gram Protocol): This is a connectionless transport layer protocol.
• Application programs that do not need connection-oriented protocol generally use UDP.

1.5 Network layer

Responsible for the routing of data (packets) through the network; handles the addressing and delivery of data. This layer provides for congestion control, accounting information for the network, routing, addressing, and several other functions. IP (Internet Protocol) is a good example of a network layer protocol. Network layer does not deal with lost messages.

Important features of Network layer protocols:

• Concerned with the transmission of packets.
• Choose the best path to send a packet (routing).
• The routing may be complex in a large network (e.g. Internet).
• Routing packets through a network may be accomplished by using simple static routes or by using complex dynamic routing algorithms.

1.6 Data link layer

Provides for the reliable delivery of data across a physical network. This layer deals with issues such as flow regulation, error detection and control, and frames. This layer has the important task of creating and managing what frames are sent out on the network. The network data frame, or packet, is made up of checksum, source address, destination address, and the data itself. The largest packet size that can be sent defines the maximum transmission Unit (MTU).

Important features of Data link layer:

• Handles errors in the physical layer.
• Groups bits into frames and ensures their correct delivery.
• Adds some bits at the beginning and end of each frame plus the checksum.
• Receiver verifies the checksum.
• If the checksum is not correct, it asks for retransmission. (Send a control message).
• Consists of two sub layers:

Logical Link Control (LLC) defines how data is transferred over the cable and provides data link service to the higher layers.

Medium Access Control (MAC) defines who can use the network when multiple computers are trying to access it simultaneously (i.e. Token passing, Ethernet [CSMA/CD]).

The data link layer provides reliable transit of data across a physical network link. Different data link layer specifications define different network and protocol characteristics, including physical addressing, network topology, error notification, sequencing of frames, and flow control. Physical addressing (as opposed to network addressing) defines how devices are addressed at the data link layer. Network topology consists of the data link layer specifications that often define how devices are to be physically connected, such as in a bus or a ring topology. Error notification alerts upper-layer protocols that a transmission error has occurred, and the sequencing of data frames reorders frames that are transmitted out of sequence. Finally, flow control moderates the transmission of data so that the receiving device is not overwhelmed with more traffic than it can handle at one time.

The protocols used in Data link layer are SLIP, PPP, MTU, and CSLP.

1.7 Physical layer

Handles the bit-level electrical/light communication across the network channel. The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems. Physical layer specifications define characteristics such as media, voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, and physical connectors.

Basically, this layer ensures that a bit sent on one side of the Network is received correctly on the other side.

Data travels from the application layer of the sender, down through the levels, across the nodes of the network service, and up through the levels of the receiver

To keep track of the transmission, each layer "wraps" the preceding layer's data and header with its own header. A small chunk of data will be transmitted with multiple layer headers attached to it. On the receiving end, each layer strips off the header that corresponds to its respective level.

Physical layer is concerned with the following:

• Physical interface characteristics like electrical, and mechanical specifications,
• Number of bits of second to be transmitted,
• Transmission type like duplex or half-duplex etc.

Frequently used Physical layer protocols:

Some of the important standards that deal with physical layer specifications are:

RS-232(for serial communication lines), X.21, EIA 232, and G730.

Physical layer and Data link layer implementations can be categorized as either LAN or WAN specifications.

• Privacy Policy
• Copyright © 2003-2024 TutorialsWeb.com

OSI Model: The 7 Layers of Network Architecture

The Open Systems Interconnection (OSI) Reference Model is a conceptual framework that describes functions of the networking or telecommunication system independently from the underlying technology infrastructure. It divides data communication into seven abstraction layers and standardizes protocols into appropriate groups of networking functionality to ensure interoperability within the communication system regardless of the technology type, vendor, and model.

The OSI model was originally developed to facilitate interoperability between vendors and to define clear standards for network communication. However, the older TCP/IP model remains the ubiquitous reference framework for Internet communications today.

Elevate your IT Service Management game plan with BMC Helix. ›

The 7 layers of the osi model.

This image illustrates the seven layers of the OSI model. Below, we’ll briefly describe each layer, from bottom to top.

1. Physical

The lowest layer of the OSI model is concerned with data communication in the form of electrical, optic, or electromagnetic signals physically transmitting information between networking devices and infrastructure. The physical layer is responsible for the communication of unstructured raw data streams over a physical medium. It defines a range of aspects, including:

• Electrical, mechanical, and physical systems and networking devices that include specifications such as cable size, signal frequency, voltages, etc.
• Topologies such as Bus, Star, Ring, and Mesh
• Communication modes such as Simplex, Half Duplex, and Full Duplex
• Data transmission performance, such as Bit Rate and Bit Synchronization
• Modulation, switching, and interfacing with the physical transmission medium
• Common protocols including Wi-Fi, Ethernet, and others
• Hardware including networking devices, antennas, cables, modem, and intermediate devices such as repeaters and hubs

2. Data Link

The second layer of the OSI model concerns data transmission between the nodes within a network and manages the connections between physically connected devices such as switches. The raw data received from the physical layer is synchronized and packaged into data frames that contain the necessary protocols to route information between appropriate nodes. The data link layer is further divided into two sublayers:

• The Logical Link Control (LLC) sublayer is responsible for flow controls and error controls that ensure error-free and accurate data transmission between the network nodes.
• The Media Access Control (MAC) sublayer is responsible for managing access and permissions to transmit data between the network nodes. The data is transmitted sequentially and the layer expects acknowledgement for the encapsulated raw data sent between the nodes.

The third layer of the OSI model organizes and transmits data between multiple networks.

The network layer is responsible for routing the data via the best physical path based on a range of factors including network characteristics, best available path, traffic controls , congestion of data packets, and priority of service, among others. The network layer implements logical addressing for data packets to distinguish between the source and destination networks.

Other functions include encapsulation and fragmentation , congestion controls, and error handling. The outgoing data is divided into packets and incoming data is reassembled into information that is consumable at a higher application level. Network layer hardware includes routes, bridge routers, 3-layer switches, and protocols such as Internet (IPv4) Protocol version 4 and Internet Protocol version 6 (IPv6).

4. Transport

The fourth layer of the OSI model ensures complete and reliable delivery of data packets.

• The transport layer provides mechanisms such as error control, flow control, and congestion control to keep track of the data packets, check for errors and duplication, and resend the information that fails delivery. It involves the service-point addressing function to ensure that the packet is sent in response to a specific process (via a port address).
• Packet Segmentation and reassembly ensure that the data is divided and sequentially sent to the destination where it is rechecked for integrity and accuracy based on the receiving sequence.

Common protocols include the Transmission Control Protocol (TCP) for connection-oriented data transmission and User Datagram Protocol (UDP) for connectionless data transmission.

As the first of three layers that deal with the software level, the session layer manages sessions between servers to coordinate communication. Session refers to any interactive data exchange between two entities within a network. Common examples include HTTPS sessions that allow Internet users to visit and browse websites for a specific time period. The Session Layer is responsible for a range of functions including opening, closing, and re-establishing session activities, authentication and authorization of communication between specific apps and servers, identifying full-duplex or half-duplex operations, and synchronizing data streams.

Common Session Layer protocols include:

• Remote procedure call protocol (RPC)
• Point-to-Point Tunneling Protocol (PPTP)
• Session Control Protocol (SCP)
• Session Description Protocol (SDP), as described here

6. Presentation

The sixth layer of the OSI model converts data formats between applications and the networks. Responsibilities of the presentation layer include:

• Data conversion
• Character code translation
• Data compression
• Encryption and decryption

The presentation layer, also called the syntax layer, maps the semantics and syntax of the data such that the received information is consumable for every distinct network entity. For example, the data we transfer from our encryption-based communication app is formatted and encrypted at this layer before it is sent across the network.

At the receiving end, the data is decrypted and formatted into text or media information as originally intended. The presentation layer also serializes complex information into transportable formats. The data streams are then deserialized and reassembled into original object format at the destination.

7. Application

The application layer concerns the networking processes at the application level. This layer interacts directly with end-users to provide support for email, network data sharing, file transfers, and directory services, among other distributed information services. The upper most layer of the OSI model identifies networking entities to facilitate networking requests by end-user requests, determines resource availability, synchronizes communication, and manages application-specific networking requirements. The application layer also identifies constraints at the application level such as those associated with authentication, privacy, quality of service, networking devices, and data syntax.

Common application layer protocols include:

• File Transfer Protocol (FTP)
• Simple Mail Transfer Protocol (SMTP)
• Domain Name System (DNS)

The internet doesn’t welcome OSI

The OSI model is widely criticized for an inherent implementation complexity that renders networking operations inefficient and slow. The academic approach to developing the OSI protocol suite relied on replacing existing protocols across all communication layers with better alternatives.

This approach failed to gain traction in the industry; vendors had already invested significant resources in TCP/IP products and had to manage interoperability with the vast choices of protocols and specifications offered by the OSI model. Additionally, academia itself considered the OSI model as an invention politically inspired by the European telecommunication and U.S. government authorities.

The older TCP/IP architecture model had already itself in real-world network environments. It served as a solid foundation for the Internet–including all of the security , privacy, and performance-related challenges. Continued research and development, investments, and industry-wide adoption of the OSI model could have made today’s cyber world a different (and perhaps better) place, but the pragmatism of the TCP/IP model gave us the internet that prevails today.

Additional resources

Learn more with these resources:

• BMC IT Operations Blog
• IT Infrastructure Management: An Introduction
• IT Infrastructure Planning: Getting Started
• Network Engineer vs Network Administrator: Roles, Responsibilities, and Job Descriptions
• What Is a Virtual Network?

Original reference image:

New strategies for modern service assurance

86% of global IT leaders in a recent IDG survey find it very, or extremely, challenging to optimize their IT resources to meet changing business demands.

These postings are my own and do not necessarily represent BMC's position, strategies, or opinion.

See an error or have a suggestion? Please let us know by emailing [email protected] .

Business, Faster than Humanly Possible

BMC works with 86% of the Forbes Global 50 and customers and partners around the world to create their future. With our history of innovation, industry-leading automation, operations, and service management solutions, combined with unmatched flexibility, we help organizations free up time and space to become an Autonomous Digital Enterprise that conquers the opportunities ahead. Learn more about BMC ›

You may also like

How to Use Automation in Event Remediation

Top IT Operations Trends in 2022

N-Modular Redundancy Explained: N, N+1, N+2, 2N, 2N+1, 2N+2, 3N/2

2020 Gartner Magic Quadrant for Privileged Access Management

IT Asset Management: Do You Know What You Have?

Why Application-Centric Infrastructure Requires A Different Approach to APM

About the author.

Muhammad Raza

Muhammad Raza is a Stockholm-based technology consultant working with leading startups and Fortune 500 firms on thought leadership branding projects across DevOps, Cloud, Security and IoT.

Computer Network

• Operating Systems
• Computer Fundamentals
• Interview Q

Physical Layer

Data link layer, network layer, routing algorithm, transport layer, application layer, application protocols, network security.

Interview Questions

• Send your Feedback to [email protected]

Help Others, Please Share

Learn Latest Tutorials

Transact-SQL

Reinforcement Learning

R Programming

React Native

Python Design Patterns

Python Pillow

Python Turtle

Preparation

Verbal Ability

Company Questions

Trending Technologies

Artificial Intelligence

Cloud Computing

Data Science

Machine Learning

B.Tech / MCA

Data Structures

Operating System

Compiler Design

Computer Organization

Discrete Mathematics

Ethical Hacking

Computer Graphics

Software Engineering

Web Technology

Cyber Security

C Programming

Control System

Data Mining

Data Warehouse

CCNA 200-301

• CCNA 200-301 Labs
• CCNP 350-401 ENCOR
• CCNP 350-401 ENCOR Labs
• CCNP 300-410 ENARSI
• CCIE Enterprise Infrastructure
• Cisco Packet Tracer Lab Course
• NRS II IRP Course
• NRS II MPLS Course
• NRS II Service Architecture
• Nokia Configuration Course
• Nokia SRC Program
• JNCIA Junos
• HCIA (HCNA)
• HCIA Configuration Course
• What is Huawei R&S Certification?
• Huawei ICT Certifications
• Python Course
• IPv6 Course
• IP Multicast Course
• NRS I Configuration Course
• Cisco Packet Tracer How To Guide
• Online Courses
• Udemy Courses
• CCNA Flashcard Questions
• Protocol Cheat Sheets
• Subnetting Cheat Sheet
• Linux Cheat Sheet
• Python Cheat Sheet
• CLI Commands Cheat Sheets
• Miscellaneous Cheat Sheets
• Cisco Packet Tracer Labs
• Cisco GNS3 Labs
• Huawei eNSP Labs
• Nokia GNS3 Labs
• Short Config Videos
• Network Tools
• IPCisco on Social Media
• Network Engineer Interview Questions
• Personality Interview Training
• Sign In/Up | Members
• Lost password
• Sign In/Sign Up
• ENROLL HERE

• OSI Referance Model

Table of Contents

OSI Model and TCP/IP Model

Computer network communication are a little complex. To understand this communication, experts divides computer network communication into different layers. To do this division, there are two standard models. These models are OSI Model and TCP/IP Model . Both of these models has different layers. OSI Model has 7 layers and TCP/IP model has 4 or 5 layers. Here, we will focus on OSI Reference Model.

As a beginner in networking, OSI Model and TCP/IP Model are very important to understand the network world. These two common standards has also been a classical question of a network engineer technical interviews . These questions are like: What are the layers of OSI Model ? How many Layers does TCP/IP Model has? What is the role of Data Link Layer?, Which devices works on Network Layer? Which protocols are there in Transport layer?  etc…

Now, let’s start with the details of OSI Model Layers . In another lesson, we will talk about TCP/IP Model and its layers detailly.

OSI Reference Model is the first term that a network engineer learn in networking. It is generally the first lesson of almost all the certification courses. It is also generally always encountered especially at the technical job interviews of junior network engineers. But OSI Model is only a visionary concept. In real network operations, you will not think about this standard and its layers.

So, what is this Referance Model? What are these OSI Layers ? Below, you can find these layers orderly from starting at the bottom, from Layer 1, to Layer 7, to the top.

Physical Layer (Layer 1)

• Data-link Layer (Layer 2) 

Network Layer (Layer 3)

Transport layer (layer 4), session layer (layer 5), presentation layer (layer 6), application layer (layer 7).

As you can see, there are 7 layers in OSI Reference Model. The first four layers are the Lower Layers and the last three layers are Upper Layers . If we compare OSI and TCP/IP Models, Lowers layers of these models has some differences, but the application layer of TCP/IP model is similar to the upper layers od OSI model.

So, whats are the roles of each OSI Model Layer ? Lets talk about them one by one detailly

Physical Layer (Layer 1) is the actual connectivity between the source and the destination. It is the layer in which the physical characteristic of the network is defined and bit flow transfer is done. These bits are determined with signals . These signals can be both analog and digital. Electrical signals are used for fixed connections and radio signals are used for wireless connections.

In other words, bit flow is done in this level, in other words meaningful 0 and 1 bits are transferred here. Basically, it is the layer in which bit streams are transferred over different media types. Network cabling , different types of cables and network connectors are on physical layer of OSI Model .  

As a simple and old network equipment, simple hubs works in physical layer  (layer 1). Because, there are not smart devices and on layer 1, there is no need for a smart device.

Below, you can find operation examples in Layer 1. The upper two are a little complex but the lower two can be a good examples:)

Data-link Layer (Layer 2)

Data-link Layer (Layer 2)  is the medium provision Layer. It defines how data is formatted for the transmission over physical medium. Layer 2 provides, node-to node connectivity. It is also the layer of physical MAC addresses .

The most important devices work on layer 2 of OSI Model is Layer 2 switches . As a network engineer, a network technician or a network admin, you will work on these data-link layer devices too much.

Data Link Layer encapsulates the data by adding a layer 2 header and layer 2 trailer . The data after adding Layer 2 Header and layer 2 trailer is called “ frame ”. Layer 2 header contains source and destination  MAC Addresses . So, it is responsible to find the next destination in the network. For decapsulation, it also removes these header and trailer and sends the data to the upper network layer.

Data Link Layer provides Error-free transfer of different categories of data frames. It detects physical layer errors and corrects them.

Different network protocols and standards are used in the different layers of OSI model. You can think them as the groups of rules. According to these protocols and standards, network technologies are used and device configurations are done.

Ethernet is one of the most popular protocol in data link layer. Ethernet is the general name of the standard which is used in our home or office networks. It defines a lot of things like cable types, data speed etc.

There are other protocols and standards in data link layers. Some of them are: PPP, HDLC, FDDI, ATM, Frame Relay.

Data-link Layer has two sub-layers . These layers are:

• Media Access Control (MAC) Layer
• Logical Link Control (LLC) Layer

The  MAC Sub Layer  controls how a computer on the network gains access to the data and permission to transmit it. And it carries the physical address of each device on the network.

The  Logical Link Control (LLC) responsible for managing frames to upper and lower layers, Flow control mechanism and Error checking.

Network Layer  is the layer of logical addressing. This layer decides the logical address of the destination with IP addresses. By doing this, it connects different networks that use different medias each other. These addresses can be IPv4 address or IPv6 address .

Network layer is also responsible for routing . With routing, it decides the path of the packets through the network. In other words, it defines the routers that the data will go through. This is one of the popular areas for network engineers :) So, routing protocols works on layer 3 (network layer).

A  Layer 3 Header  (IP Header for IP) is added to the data in this layer. After this addition, frame is called as “ packet ”. In this header also the next level protocol is mentioned. It can be any  transport layer protocol like,  TCP , UDP and SCTP .

Frame fragmentation is also done in this Layer. If a frame is longer than  MTU   (Maximum Transmission Unit) , then it is fragmented (divided to the little packets).

There are different protocols used in network layer of OSI. Some of these protocols are IPv4, IPv6, ICMP, IPSec, OSPF, BGP etc.

Transport Layer (Layer 4)  establishes, maintains and manages end to end secure connection. This layer allows multiple transports on same physical connection.

Transport layer is responsible for end-to-end Error Recovery and Flow Control . Transport Layer is also the Layer that QoS facilities also performed.

Different transport layer protocols are used in this layer. According to used Layer 4 protocol, Layer 4 Header is added to the packet in this Layer. After this the packet called “ segment ”.

There are two common Transport Layer Protocols. These Protocols are  UDP (User Datagram Protocol) and  TCP (Transmission Control Protocol) . There are also other  Transport Layer Protocols  like SCTP,  RSVP etc. But in CCNA, we will focus on TCP and UDP. We will explain the details of these protocols in the following lessons.

Below, you can find some of TCP layer 4 operations and transport layer protocols comparison.

Session Layer (Layer 5)  is the OSI layer, that responsible for session jobs between the end points. It provides session establishment, maintenance and termination. Logging is also done in this layer. One of the other main roles of this session layer is multiplexing transport layer services.

Session layer is mainly used by applications themselves. So, network engineers do not work on this layer. This is the job of application developers.

Log in to a computer for file transfer can be an example of session layer job.

Presentation Layer (Layer 6)  is the  Translator  of the network. There are many data formats in any network. This layer provides translation between different formats.

For example, image formats like jpeg, gif, voice formats like mp3 and video format like mp4 are all need different behaviors. Presentation layer converts this different data into a readable format.

Encryption and compression of the data are also done in presentation layer (layer 6) of OSI Model.

Application Layer (Layer 7)  is the closest layer to user. It is the interface between applications and the underlying Layers. It provides the provision of the communication between Applications. Application Layer let programs to reach different network resources.

Some of the  protocols  used in Application Layer are  Telnet, FTP , SMTP, HTTPS, SSL  etc. Different services work with these protocols. For example, the browser that you use internet, works with HTTP.  By the way, the application itself is not belong to this layer.

• Network Devices

As you know in a network there are some common network devices like Hubs, Repeaters, Bridges, Switches and Routers. These devices operates in different Layers of OSI model. Lets check this devices and the Layers they are used.

Everything begins with the data at the Application Layer. Then through out the destination, in each Lower Layer, an additional Header is added to these Data. And at the Physical Layer, all these are send via 1s and 0s as voltages over the cables.

What are these Headers of OSI Model Layers and how effects the data?

• At Transport Layer , Layer 4 Header is added. Here the name of “Data and Layer 4 Header” is “ Segment ”.
• At Network Layer , Layer 3 Header is added. Here the name of “Data and Layer 3 Header” is “ Packet ”.
• At Data-Link Layer , Layer 2 Header is added. Here the name of “Data and Layer 2 Header” is “ Frame ”.
• At Physical Layer , all of them is translate to 1s and 0s as voltages.And these values are sent and received over different media types(cables).

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

Network Fundamentals

• Collision Domain vs Broadcast Domain
• Coaxial Cable Details
• Types of Networks
• Top Internet Access Technologies
• WAN Topology Types
• Network Topology Architectures
• Power Over Ethernet (PoE)
• Ethernet Collisions and Troubleshooting
• Cisco NGFW and Cisco NGIPS
• Networking Connectors
• Ping Command
• Basic Cisco Router Configuration on Packet Tracer
• ICMP (Internet Control Message Protocol)
• Address Resolution Protocol (ARP)
• Network Cabling
• TCP/IP Model

IPv4 Addressing

• Verify IP Parameters for Client OS
• IPv4 vs IPv6 Comparison
• Cisco IP Address Configuration
• APIPA Address
• Private IP Address Ranges
• Subnetting Examples
• IP Addressing (IPv4)
• IP Subnetting and Subnetting Examples

TCP and UDP

• TCP Header : Sequence & Acknowledgement Number
• TCP Handshake
• TCP versus UDP
• UDP (User Datagram Protocol)
• TCP (Transmission Control Protocol)
• TCP Header : TCP Options
• TCP Header : TCP Window Size, Checksum & Urgent Pointer
• TCP Header : TCP Flags
• Voice VLAN Configuration
• Packet Tracer VLAN Example 2
• How to Configure Cisco VLANs
• VTP Configuration with Packet Tracer
• VTP (VLAN Trunking Protocol)
• DTP and VLAN Frame Tagging protocols ISL, dot1.q
• Cisco Packet Tracer VLAN Configuration Example
• VLAN Port Assignment and VLAN Port Types
• VLANs (Virtual Local Area Networks)

Switching and LANs

• Ethernet Basics
• Cisco Switch Configuration on Cisco Packet Tracer
• MAC Address Lookup
• What is a mac address
• Local Area Networks
• Network Topologies
• Hubs, Switches and Routers

Spanning Tree Protocol

• Loop Guard, Uplink Fast, Backbone Fast and UDLD
• Portfast, Root Guard, BPDU Filter and BPDU Guard
• PVST+ and Rapid PVST+
• STP (Spanning Tree Protocol) Example on Packet Tracer
• RSTP Configuration on Packet Tracer
• STP Portfast Configuration with Packet Tracer
• Spanning Tree Protocol Operation
• Rapid Spanning Tree Protocol (RSTP)
• Spanning Tree Protocol (STP)

Neighbor Discovery

• LLDP Configuration on Cisco IOS
• Neighbour Discovery Protocols
• CDP Configuration with Packet Tracer

EtherChannels

• PAgP Configuration on Cisco Devices
• LACP Configuration on Cisco Devices
• Link Aggregation Control Protocol (LACP)

Routing Fundamentals

• Route Summarization
• Routing Path Determination
• Routing Table
• Static Routes
• IPv4 Floating Static Routes
• Inter VLAN Routing Configuration on Packet Tracer
• Switch Virtual Interface Configuration on Packet Tracer
• Switch Virtual Interfaces
• Inter VLAN Routing with Router on Stick
• IP and Layer 3 Overview
• Static Route Configuration on Cisco Routers
• Dynamic Routing Protocols
• OSPF Cost and SPF Algorithm
• OSPFv3 Configuration Example on Cisco IOS
• OSPFv3 (Open Shortest Path First Version 3)
• Cisco Single Area OSPF Configuration
• Other OSPF Key Points
• OSPF Network Types
• OSPF Area Types
• OSPF LSA Types
• OSPF Packet Types
• OSPF Adjacency
• OSPF(Open Shortest Path First) Overview

WAN (Wide Area Networks)

• MLPPP Configuration on Cisco Packet Tracer
• What is MLPPP?
• Metro Ethernet Technology
• WAN and WAN Technologies

DHCP and DNS

• DNS Configuration on Cisco Routers
• Domain Name System Overview
• Router DHCP Configuration with Packet Tracer
• DHCP IP Allocation Operation
• DHCP (Dynamic Host Configuration Protocol)
• NAT (Network Address Translation)
• PAT Configuration with Packet Tracer
• Dynamic NAT Configuration with Packet Tracer
• Static NAT Configuration with Packet Tracer

First Hop Redundancy

• HSRP Configuration on Cisco IOS
• HSRP (Hot Standby Router Protocol)
• First Hop Redundancy Protocols (FHRPs)

Network Management

• Syslog Overview
• Configuration Register
• TFTP, FTP, SFTP and SCP
• SSH Configuration on Packet Tracer
• Syslog Configuration Cisco
• Cisco NTP Configuration
• NTP (Network Time Protocol)
• SNMP Overview
• SNMP Configuration On Cisco IOS
• Cisco Router Password Recovery
• IPv6 Floating Static Routes
• IPv6 Static and Default Route Configuration
• Stateless Address Auto Configuration
• IPv6 NDP (Neighbour Discovery Protocol)
• IPv6 Configuration on Cisco Packet Tracer
• What does IPv6 bring?
• Subnetting in IPv6
• IPv6 Address Types
• IPv4 and IPv6 Headers
• IPv6 and IPv6 Addresses

Quality of Service

• Network Traffic Types
• Policing and Shaping in QoS
• Classification and Marking in QoS
• Quality of Service Overview
• Radius Configuration for Wireless Users
• Cisco RADIUS Server Configuration on Packet Tracer
• TACACS+ Overview
• RADIUS Overview
• AAA Protocols : RADIUS and TACACS+
• Authentication, Authorization, Accounting (AAA)
• WLAN Frequency Bands
• Other Wireless Network Extention Types
• Wireless Principles
• WLAN Components
• Wireless Network Design Models
• WLC Management Access Connections
• Wireless Access Point Modes
• Wireless Security Protocols
• WLAN Configuration on Packet Tracer

Security Fundamentals

• DHCP Snooping Configuration on Packet Tracer
• Cisco Banner Configuration on Packet Tracer
• What is DHCP Snooping?
• Access Control Lists
• Multifactor Authentication (MFA)
• Dynamic ARP Inspection
• Cyber Attacks, Network Attacks, Threats and Mitigation
• 802.1x (Port Based Network Access Control)
• Switch Port Security Configuration on Cisco Packet Tracer
• Switch Port Security
• Extended Access List Configuration With Packet Tracer
• Standard Access List Configuration With Packet Tracer
• Basic Cisco Router Security Configuration

Automation and Programmability

• Ansible vs Puppet vs Chef
• Chef Overview
• Puppet Overview
• Ansible Overview
• Network Automation Tools
• Interpret JSON Encoded Data
• Cisco DNA Center
• Cisco SD-Access
• Data Serialization Languages: JSON, YAML, XML
• Traditional Network Management versus Cisco DNA Center
• Cisco DNA and Intent-Based Networking (IBN)
• How Network Automation Impacts Network Management

SDN (Software Defined Networking)

• What is SDN ?
• Traditional Network Drawbacks Versus SDN
• What Will SDN Bring?
• SDN Architecture Components
• SDN Terminology
• Virtualization
• Virtual Network Structure

Latest Lessons

• Coaxial Cable Details Part of: CCNA 200-301
• IPv4 vs IPv6 Comparison Part of: CCNA 200-301
• IGMPv3 Overview Part of: IP Multicast Course
• IGMPv3 Overview Part of: CCIE Enterprise Infrastructure
• Subnetting IPv6 Part of: CCIE Enterprise Infrastructure
• IPv6 and Subnetting Part of: CCNP Enterprise 350-401 ENCOR
• IPv6 Link Local Address Type Part of: CCIE Enterprise Infrastructure
• Cisco Switch Configuration on Cisco Packet Tracer Part of: CCNA 200-301
• Switch Configuration on Cisco Packet Tracer Part of: Cisco Packet Tracer Lab Course
• REST API Security Part of: CCNP Enterprise 350-401 ENCOR
• More Lessons

Latest Blog Posts

WHAT YOU WILL FIND?

• 250.000+ Students All Over The World
• 8.000+ Questions & Answers
• 100+ Lab Files & Cheat Sheets
• 30+ IT/Network Courses
• A Real Desire To Help You
• Daily Social Media Shares
• %100 Satisfaction
• CISCO Courses
• NOKIA Courses
• HUAWEI Courses
• JUNIPER Courses
• PYTHON Course
• KEY Courses
• VIDEO Courses
• UDEMY Courses
• Cheat Sheets
• Configuration Files
• Interview Questions
• IPCisco On Social Media
• Pärnu mnt. 139c – 14, 11317, Tallinn, Estonia
• [email protected]

Create your address on the web.

• Domain Check

Move your domain name to IONOS.

• Free Domain

Secure site traffic and build trust.

Create your own website easily.

Create your own online store.

Fast, scalable hosting for any website.

Optimized for speed, reliablity and control.

Reach out with your own email address.

Secure and share your data on the go.

Powerful Exchange email and Microsoft's trusted productivity suite.

Pay as you go with your own scalable private server.

• Virtual Private Servers (VPS)

Get enterprise hardware with unlimited traffic

Individually configurable, highly scalable IaaS cloud

• Business Name Generator
• Logo Creator
• Favicon Generator
• Whois Lookup
• Website Checker
• SSL Checker
• IP Address Check

What is the presentation layer in the OSI model?

The presentation layer is the sixth layer in the OSI model and is responsible for converting different file formats. This allows two systems to communicate. Other tasks carried out by the sixth layer include data compression and encryption.

What is the presentation layer?

What does the presentation layer do, which format does the presentation layer use, presentation layer protocols, skipping the presentation layer.

The presentation layer is the sixth layer of the OSI model. It is primarily used to convert different file formats between the sender and the receiver . The OSI model is a reference model that is used to define communication standards between two devices within a network . The development of this standard began in the 1970s and it was first published at the beginning of the following decade. This standard enables seamless interaction between different technical systems.

The model is made up of a total of seven different layers, all having their own clearly defined tasks. While there are clear boundaries between the layers, the layers interact with each other, with each layer building off the one below it. The different layers are as follows:

• Physical layer
• Data link layer
• Network layer
• Transport layer
• Session layer
• Presentation layer
• Application layer

The presentation layer interacts closely with the application layer, which is located directly above it. The presentation layer’s main task is to present data in such a way that it can be understood and interpreted from both the system sending the data and the system receiving it. After this has been accomplished, the application layer then determines how the data should be structured and what sort of data and values are permissible.

Using these entries, a command set, or an abstract transfer syntax, is then automatically created. The presentation layer now has the task of transferring the data in such a way that it is readable without changing the information contained within it.

The presentation layer is often also responsible for the encryption and decryption of data . The information is first encrypted on the sender’s side and then sent to the receiver in an encrypted state. Keys and encryption methods are then exchanged in the presentation layer. The recipient is then able to decrypt the unreadable data and convert it into a format that can be understood and interpreted.

If data is shown during a transfer, we often use the term transfer syntax. These are separated into the abstract transfer syntax , in which the transferred values are written, and the concrete syntax, which contains a definition of the value coding.

The receiver can only process and understand the data they receive if they receive all of the information from the presentation layer. The most common definition language is Abstract Syntax Notation One (ASN.1) , which is also recommended by the ISO. The ISO is an organization that is responsible for developing international standards in technology, management and manufacturing.

The presentation layer has many different formats. The most common text formats are the ASCII (American Standard Code for Information Interchange) and EBCDIC (Extended Binary-Coded Decimal Interchange Code). The most common image formats are GIF, JPEG and TIFF. Widely used video formats include MIDI, MPEG and QuickTime.

There are many different presentation layer protocols as well as transfer and encryption technologies in the presentation layer. These include:

The tasks which are carried out by the presentation layer are not always necessary for communication between two systems. In instances where both systems use the same formats, data conversion is not necessary. Additionally, encryption and compression are not required for every interaction and can also be carried out in another layer of the OSI model. If this is the case, the presentation layer can be skipped and the application layer (7) can communicate directly with the session layer (5) instead .

• Encyclopedia

Build or host a website, launch a server, or store your data and more with our most popular products for less.

Want to create or adapt books like this? Learn more about how Pressbooks supports open publishing practices.

6 The Network Layer I | Addressing

Chapter objectives.

• Describe the purpose of the Network Layer .
• Differentiate between the data and control planes of the Network Layer.
• Summarize the components of an IP packet header .
• Explain the IPv4 classes and their use in different types of networks.
• Describe the purpose of subnets and subnet masking .
• Determine the allocation of IP addresses across a network using subnet masking.
• Explain the function of Network Address Translation (NAT).
• Summarize the purpose and features of IPv6 .

The Network Layer | Addressing

As noted in the overview, the OSI model layer built above the data link layer is the network layer. Similar to frames on the data link layer, its method of data exchange is in the form of a datagram . The datagram contains source and destination information as it passes through several intermediate devices called routers on its way from its source to its destination. We will discuss this information in this section, and how it is routed in the next.

The Data Plane and the Control Plane

The network layer operates on two planes, the data plane and the control plane . Consider taking a trip by plane. The data plane acts as the plane that transports passengers to the correct destination (or forwards the passengers). The control plane works as the traffic controller that controls and routes traffic.

The data plane performs tasks like receiving and inspecting the packets, forwarding them to the correct destination, queuing network packets during network congestion, and ensuring the safe delivery of packets to the correct destination. It accomplishes these tasks by leveraging the Internet Protocol and Ethernet. The data plane operates at a lower abstraction level than the control plane — forwarding packets based on the routing configured by the control plane.

The control plane is responsible for determining the “best” routing and path configuration, with protocols such as OSPF, RIP, and BGP. These enable the control plane to exchange routing details for optimal network traffic paths. The control plane typically operates separate from the data plane, allowing for greater efficiency and scalability. It is also distributed and centralized. We’ll return to the control plane in the next chapter.

The IP Datagram

As you recall, a packet is also called a datagram, a segment, a block, a cell, or a frame depending on the protocol used for the transmission of data. In this section, we’ll use the term datagram . The IP header is a crucial component of the IP protocol that is used to transport data across the Internet. This header is a part of the datagram that is transmitted over the Internet and contains important information about the source and destination of the data.

When data has to be transmitted, it is broken down into similar structures of data before transmission. Once they reach their destination the pieces are reassembled into the original data chunk.

The structure of a datagram depends on the type of packet it is and on the protocol. Normally, packets have a header and a payload .

The header keeps overhead information about the packet, the service, and other transmission-related data. For example, data transfer over the Internet requires breaking down the data into IP packets, as defined in the IP (Internet Protocol). An IP packet includes:

• The source IP address , which is the IP address of the machine sending the data.
• The destination IP address , which is the machine or device to which the data is sent.
• The sequence  number  of the packets, a  number  that puts the packets in order such that they are reassembled in a way to get the original data back exactly as it was prior to transmission.
• The type of service
• Any relevant flags
• Some other technical data
• And finally, the payload , which represents the bulk of the packet (all the above is considered as overhead), and contains the actual data being transmitted. This can be any type of data, such as text, images, or video . The payload is usually encrypted for security purposes, especially when sensitive data is being transmitted.

Adapted from: https://www.lifewire.com/what-is-a-data-packet-3426310

Attribution: Michel Bakni, CC BY-SA 4.0  https://creativecommons.org/licenses/by-sa/4.0 , via Wikimedia Commons

The IP datagram is an essential component of the TCP/IP suite, which is used for communication on the internet. Without IP datagrams, computers would not be able to communicate with each other over the Internet. IP datagrams are used in conjunction with other protocols, such as TCP and UDP, to ensure that data is transmitted reliably and efficiently.

To see how a packet travels the Internet, watch this video from the World Science Festival [3:29]:

IPv4 Addressing

The network layer enables the transmission of information between hosts that are not directly connected through intermediate routers. To send one byte of information to host  B , host  A needs to place this information inside the datagram. An IP address is a numeric identifier assigned to each machine on an IP network. It designates the specific location of a device on the network.

An important issue in the network layer is the ability to identify a node (host or router) inside the network. This identification is performed by associating an address to each node. An address is usually represented as a sequence of bits. Most networks use fixed-length addresses. At this stage, let us simply assume that each node in a network has an address that corresponds to the binary representation of its name.

Let’s review some important terminology associated with an IPv4 (Internet Protocol Version 4) address.

• Bit – A bit is one digit, either a 1 or a 0.
• Byte – A byte is 8 bits.
• Octet – An octet, made up of 8 bits, is just an ordinary 8-bit binary number.
• Network address – This is the designation used in routing to send packets to a remote network – for example, 10.0.0.0, 172.16.0.0, and 192.168.1.0.
• Broadcast address – The address used by applications and hosts to send information to all nodes on a network is called the broadcast address. Examples include 255.255.255.255, which is all networks, all nodes; 172.16.255.255, which is all subnets and hosts on network 172.16.0.0; and 10.255.255.255, which broadcasts to all subnets and hosts on network 10.0.0.0.

An IP address consists of 32 bits of information. These bits are divided into four sections , referred to as octets or bytes, each containing one byte (8 bits). You can depict an IP address using one of these methods:

• Dotted-decimal, as in 172.20.18.125
• Binary, as in 10101100.00010100.00010010.01111101

The 32-bit IP address is a structured or hierarchical address, as opposed to a flat or nonhierarchical address. Although either type of addressing scheme could have been used, hierarchical addressing was chosen for a good reason. The advantage of this scheme is that it can handle a large number of addresses, namely 4.3 billion (a 32-bit address space with two possible values for each position – either 0 or 1 – gives you 2 32 , or 4,294,967,296). Each octet can have a value from 0 to 255 in decimal notation, or from 00000000 to 11111111 in binary notation. For example, the address 192.168.0.1 has the following binary representation: 11000000.10101000.00000000.00000001.

The network address (which can also be called the network number) uniquely identifies each network. Every machine on the same network shares that network address as part of its IP address. In the IP address 172.20.18.125, for example, 172.20 is the network address. The node address, also referred to as a host address, is assigned to, and uniquely identifies, each machine on a network. This part of the address must be unique because it identifies a particular machine, that is, an individual node, as opposed to a network, which is a group. In the sample IP address 172.20.18.125, the 18.125 is the node address. The designers of the Internet decided to create classes of networks based on network size. For the small number of networks possessing a very large number of nodes, they created the rank Class A  network. At the other extreme is the  Class C  network, which is reserved for the numerous networks with a small number of nodes. The class distinction for networks between very large and very small is predictably called the  Class B  network.

More specifically, IPv4 addresses are classified into five classes: A, B, C, D, and E. Each class has a different range of values for the first octet, and a different default subnet mask . The classes are:

• Class A : The first octet ranges from 1 to 126, and the default subnet mask is 255.0.0.0. This means that there can be up to 126 networks, each with up to 16 million hosts. For example, the address 10.0.0.1, reserved for the class A space, specifies “10.” as the network portion and “.0.0.1” as the host portion.
• Class B : The first octet ranges from 128 to 191, and the default subnet mask is 255.255.0.0. This means that there can be up to 16 thousand networks, each with up to 65 thousand hosts. For example, the address 172.16.0.1 belongs to class B, and specified the network part as “172.16.” and the host part as “.0.1”.
• Class C : The first octet ranges from 192 to 223, and the default subnet mask is 255.255.255.0. This means that there can be up to 2 million networks, each with up to 254 hosts. For example, the address 192.168.0.1 belongs to class C, and specified the network part as “192.168.0.” and the host part as “.1”.
• Class D : The first octet ranges from 224 to 239, and these addresses are reserved for multicast purposes, which means they are used to send data to multiple hosts at once.
• Class E : The first octet ranges from 240 to 255, and these addresses are reserved for experimental purposes.

By definition, an IP address that begins with 8 in the first octet is in a Class A network, so the network part of the address is the first byte, or first octet. An address that begins with 130 is in a Class B network. By definition, Class B addresses have a 2-byte network part, as shown. Finally, any address that begins with 199 is in a Class C network, which has a 3-byte network part. Also by definition, a Class A address has a 3-byte host part, Class B has a 2- byte host part, and Class C has a 1-byte host part.

Some special types of IPv4 addresses have different functions or meanings:

• Loopback address : This is the address 127.x.x.x (usually written as  127  . 0  . 0  . 1  ), which is used to test the connectivity of a device with itself.
• Broadcast address : This is the address that has all bits set to  1  in the host part (usually written as x.x.x ** .255** ), which is used to send data to all hosts on a network.
• Network address : This is the address that has all bits set to  0  in the host part (usually written as x.x.x ** .0** ), which is used to identify a network as a whole.
• 10  .x.x.x (class A)
• 172  . 16 .x.x –  172  .31.x.x (class B)
• 192   .168 .x.x (class C)

IP Subnetting

IPv4 addresses are used to identify both networks and hosts on the internet. A network is a group of devices that can communicate with each other, and a host is a device that has an IP address assigned to it. To distinguish between the network and host parts of an address, a subnet mask is used. A subnet mask is another 32-bit number that has 1s in the network part and 0s in the host part. For example, the subnet mask 255.255.255.0 has the following binary representation: 11111111.11111111.11111111.00000000.

IP subnetting creates vastly larger numbers of smaller groups of IP addresses compared with simply using Class A, B, and C conventions. The Class A, B, and C rules still exist, but with subnetting, a single Class A, B, or C network can be subdivided into many smaller groups. Subnetting treats a subdivision of a single Class A, B, or C network as if it were a network itself. By doing so, a single Class A, B, or C network can be subdivided into many nonoverlapping subnets. The following figure shows two host networks and the server network, wikibooks.org.

With subnetting, the third part of an IP address-namely, the subnet-appears in the middle of the address. This field is created by “stealing” or “borrowing” bits from the host part of the address. The size of the network part of the address never shrinks. In other words, Class A, B, and C rules still apply when you define the size of the network part of an address. The subnet mask tells us how many bits are used for the network part and how many bits are used for the host part of an address. The more bits are used for the network part, the fewer bits are available for the host part, and vice versa. This affects how many networks and hosts can be created with a given address range.

However, the host part of the address shrinks to make room for the subnet part of the address, as shown below.

Constructing an IP Network Addressing Scheme

We can now consider best practices in allocating IP addresses. We are going to describe classful and classless operations, including use of subnets. Using real-life examples we will describe the process of calculating subnet host addresses. In doing so, we will describe the use of subnet masks, and how they are used by routers.

Flat Topology

When talking about routing one tends to think about forwarding packets to remote destinations. The immediate thought is WAN and the Internet, however, routing also makes sense in a campus network or smaller local area networks for the purposes of traffic segmentation. If we do not have routing, then we are talking about one flat network or flat topology where all devices belong to the same logical segment. A logical segment is a broadcast domain where, in a flat topology, all devices share the same broadcast domain and receive each others’ broadcasts. This design impacts performance and throughput in the network. The only intelligence in filter mechanism would be a Layer 2 switch, which forwards based on MAC addresses. MAC addresses have no hierarchical structure and therefore, we are still talking about a flat network. The more machines and devices you add, the more performance degradation you are going to experience. Routers can be used in these scenarios to break the network into multiple broadcast domains or subnets.

Subnetworks

Advantages to subnets are not only on the performance side. With the segmentation in multiple subnets, overall traffic is reduced and each subnet is a broadcast domain.

A router is effective at stopping local broadcasts, but there are more advantages available with subnetting. You are breaking things into smaller pieces and this divide-and-conquer approach will make things easier to manage. By compartmentalizing the network you can apply different policies to each individual compartment. The router would be the overall policy in force because it controls traffic from one subnet to the other, effectively isolating network problems. If something happens on one subnet the effect is mitigated by the router in other subnets.

Security is also enhanced with subnetting. A denial of service attack may not reach other subnets if the router is acting as some sort of a firewall in the middle, regardless of the reason for subnetting. We would need to assign each subnet a different network or subnet ID and then hosts within that subnet would have a unique host ID portion of the IP address.

What a Subnet Mask Does

How can we tell which portion of the IP address is the network and which portion is the host? In classful routing, the class would tell us which bytes are dedicated to the network ID and which bytes are dedicated to the host. When we subnet a classful IP address, what we are doing is stealing bits from the host portion of the address and then making them part of the subnet portion of the address. We are effectively creating a third leg of the hierarchy. We are going to have the network defined by the class, but also a subnet and also then the host. We are dividing networks into subnets and then subnets contain hosts.

This is a hierarchy that is similar to our telephone numbering system; we have country codes and then city codes and then telephone numbers. Telephone numbers require the city code outside a city, and a country code outside a country to provide a unique number. When we do this, we are talking about a classless environment. In classless routing, the class of the address no longer tells us what portion of the address is network, subnet, or host. It is in the subnet mask , the one component that will tell us each section. In this context, then the mask is not similar to the Halloween masks that your kids may use. This is a mask that serves as a pair of glasses to look at the addresses differently. It is a measuring tool that tells you how far to look into the IP address to find the network piece, then the subnet piece, and then what is left is the host piece.

Possible Subnets and Hosts for a Class C Network

Classless Inter-Domain Routing is also called CIDR , and that’s what we will discuss next. The subnet mask is used by hosts to identify traffic that travels outside of their own subnet. It is also used by routers to identify networks and subnets to forward traffic toward those destinations. In more specific terms, it is nothing more than a tool for borrowing bits, the example here is a class C network. The classful paradigm tells us that the first three bytes of the address represent the network portion, whereas the last byte represents the host portion. Well, when subnetting, we want to create subnets out of the network; the network bytes are fixed and so we are going to need to use some of the bits dedicated to the host as subnet bits. One general rule here is that we are borrowing bits; in other words, the more bits we take from the host portion, the fewer hosts we are going to be able to represent, but the more subnets we will have. Consequently, the number of subnets and the number of hosts depends on the number of bits we borrow. When these bits are borrowed, the quantity of subnets possible will be 2 to the power of the number of bits borrowed.

Network.Network.Network.Host

In a class C address we will call the number of borrowed bits  S  and the number of hosts  H , where  H  is nothing more than  8-S  (8 bits minus the number of bits borrowed). Again, the more subnet bits we borrow, the fewer hosts we are going to have and the total bits we have available to borrow in a class C is 8. With 1 bit, we are going to be able to represent 2 to the first power as a number of subnets, or two subnets. That leaves us with the 7 bits for the host, which equates to 126 possible hosts per subnet. Notice that 2 to the seventh power is 128, but we do have 2 reserved addresses, all 0s and all ls. Neither can be used as a host address because those are reserved; they represent the network itself and the broadcast. You can reach similar conclusions if you increase the number of bits borrowed. The more bits you borrow, the more subnets you will have, the fewer hosts you will have.

Possible Subnets and Hosts for a Class B Network

In this class B example, we have more room to borrow bits from the host portion. But it follows the similar process and a similar logic. In this example, we borrow, let us say, 4 bits. That means we have room for 16 subnets out of the class B network, leaving us with 12 bits for the host. This time we have 16 subnets available minus 4 bits borrowed leaves 12 bits for the host. Raising 2 to the 12th power gives us 4096 possible hosts. Again, we have to subtract the two reserved addresses giving us 4094 hosts for each one of the four subnets.  

Network.Network.Host.Host

Possible Subnets and Hosts for a Class A Network

The same goes for class A addresses. This class is probably the most flexible one in terms of subnetting because we have a lot more bits to borrow, explaining why one of the most popular IP addresses or network addresses is the class A 10.x.x.x private address block. With that, you can actually borrow the whole second byte to represent your subnets and still you have a few thousand hosts available per subnet.

Network.Host.Host.Host

End System Mask Operation

End systems and hosts will use the subnet mask to identify the network that they are located at in terms of the IP hierarchy. They will then compare that network with the destination address of their packets. If the destination matches their own network according to the mask, they will try to send an ARP request to obtain the MAC address of the destination. It will then forward a packet straight to the destination in the local subnet.

If the networks do not match on the other hand, then that means the destination is another remote network and they will need to resolve for the router’s IP address. The router is the component that will take them to or forward their packets to the remote destination. The router’s IP address is nothing more than the default gateway configured in each one of the machines.

How Routers Use Subnet Masks

The function of the subnet mask is the same in the case of routers, but the routers are going to use the information differently. They will receive packets in understanding that they are responsible for forwarding them to the intended destination. They will use the mask to compare the destination IP with the known destinations in the routing table. In this example, the naming convention for the subnet mask is “/” followed by the number, where the number tells the router which bits are the network portion of the address.

A /24 means that the first 3 bytes or 24 bits are the network portion of the address. Also, in this example, host A is trying to send packets to destination 10.3.1.23; that host will use the mask /24 to compare its own subnet ID. Using the subnet mask, the host will compare is subnet ID (10.1.1) with the first 3 bytes of the destination subnet ID (10.3.1). They do not match, so host A will send the packet to the router. The router then will use the /24 mask to compare the first three bytes of the received packet’s destination IP to match the destination subnet to the entries in the routing table. It will find that there is an entry that matches the destination. Not only that, but it follows that the outgoing interface to forward that packet through is fa0/0. That is how the packet reaches router B, which follows the same procedure and delivers the packet to the directly connected subnet.

Applying the Subnet Address Scheme

When subnetting the network, we need to make sure of several things. First, we need to plan our subnetting scheme and our subnetting strategy to steal as many bits from the host portion of the address as needed to represent the number of subnets and hosts. When that design portion is complete, then we need to allocate and assign the subnets to actual network segments. The subnet mask is a tool that will tell all devices, hosts, and routers how to read the destinations and how to forward packets accordingly.

Octet Values of a Subnet Mask

We know why we need subnet masks, but what does one look like and how do we build them and design them? Well, a subnet mask will be nothing more than a 4-byte word similar to an IP address. In other words, it is a string of 32 bits, 1s or 0s. Essentially, the subnet mask looks like an IP address, but is not. Instead, it is sent along with the IP address to allow you to identify the host portion of the address, the subnet ID, and network portions of the address. In that sense, all 1s in the subnet mask indicate that the corresponding bits of the IP address are part of the network portion of the address. All 0s in the subnet mask indicate that the corresponding bit of the IP address will be part of the host portion of the address.

Now remember, we said that subnet masks are nothing more than a borrowing mechanism. We will shift the default mask to the right, borrowing bits from the host portion of the address. Doing so results in one of the key features of the subnet mask which is that the 1s are always consecutive and so are the 0s.

As we move down this diagram, this makes sense in terms of our knowledge of the binary to decimal conversions. I n the end, all 1s in a subnet mask constitute the binary representation of the decimal number 255. This number is the greatest subnet mask, or the representation of the subnet mask in decimal numbers, for any byte.  That is why 24 consecutive bits set to 1 results in the subnet mask 255.255.255.0.

Default Subnet Masks

In the case of a class A address, the 10.0.0.0 is the default subnet mask, and the one that tells us how the address class identifies the network portion of the address. Specifically, the first octet is all 1s and the rest of the three bytes are 0s. This corresponds to 255.0.0.0 in decimal notation. What is the /8 then? Instead of typing 255.0.0.0, designating /8 will give us a mask with 8 consecutive bits set to one and the rest are 0s. This highlights another feature of subnet masks.

When we subnet, we always start from the left and move to the right in defining the number of bits set to 1 that will give us the network and subnet IDs. Knowing that a subnet mask has 32 bits, all we need to know is how many bits are set to 1, because with that we know that the rest of the bits will be a consecutive string of 0s. Something similar happens in classes B and C. For class B, 172.16.0.0, is the default mask, the one that tells us that the class one is 2 bytes set to 1 that is 255.255.0.0 or /16. The same thing is true for class C addresses with that default mask, which is nothing more than a /24.

Procedure for Implementing Subnets

It is now time to look at this as a process in which we obtain an IP address from a registry authority like IANA or Internet Assigned Numbers Authority and then split it into multiple subnets. We get one network, but we will need more than one, because most likely we do not have a flat structure or topology. The number of segments in our network will give us the number of subnets we need. It is time then to define a new subnet mask that extends the default mask to the right. Well, we know that the subnet mask is nothing more than a borrowing tool and so we will go ahead and calculate the number of bits we need to borrow in order to represent the number of subnets required.

Remember that every bit we borrow from the host will take away from the host number . The more subnets we have, the fewer hosts per subnet. With that in mind we will reach a new subnet mask. It will most likely be a string of 1s and 0s, rather consecutive 1s followed by consecutive 0s. It will be our job to make it a decimal value and represent it in dotted decimal notation. With the new mask, it is now time to define the subnets. In other words, it is similar to creating or generating area codes. If we compare the network that was given by the registry authority to a country code, the subnets we create will be equivalent to the area codes. The hosts that fall within each subnet would then be the specific phone numbers. Here is an example: +1 (214) 123-4567 or +Network (Subnet) Host-Address. Each network segment will be allocated a different unique subnet ID within the network and then each host will need to have unique host identifiers within the subnet to create our hierarchy. When the planning is done, it is time to go into each host, device, and router and apply the IP addressing scheme with the appropriate mask.

This section is adapted (Bingman, Mitra) from “IP Network Addressing Scheme | Class A B C Subnets.” Learncisco.Net, https://www.learncisco.net/courses/icnd-1/lan-connections/network-addressing-scheme.html. Accessed 16 Sept. 2023. Except where otherwise noted, content on this site is licensed under a  Creative Commons Attribution 4.0 International license .

Wrap up your understanding of IPv4 addressing and subnetting with this CertBros video [17:54]. Then move onto to NAT!  

Network Address Translation (NAT)

Whether your network is the home or the corporate type, if it uses the private IP addresses, you have to translate your private inside addresses to a global outside address by using NAT. The main idea is to conserve Internet global address space, but it also increases network security by hiding internal IP addresses from external networks . In NAT terminology, the inside network is the set of networks that are subject to translation. The outside network refers to all other addresses – usually those located on the Internet. However, it’s important to understand that you can translate packets coming into the private network as well. NAT operates on a router – generally only connecting two networks together – and translates your private (inside local) addresses within the internal network, into public (inside global) addresses before any packets are forwarded to another network. This functionality gives you the option to configure NAT so that it will advertise only a single address for your entire network to the outside world. Doing this effectively hides the internal network from the whole world really well, giving you some much-needed additional security.

There are different flavors of NAT:

• Static NAT – Designed to allow one-to-one mapping between local and global addresses. This flavor requires you to have one real Internet IP address for every host on your network.
• Dynamic NAT – Designed to map an unregistered IP address to a registered IP address from out of a pool of registered IP addresses. You don’t have to statically configure your router to map an inside to an outside address as in static NAT, but you do have to have enough real IP addresses for everyone who wants to send packets to and from the Internet.
• Overloading – This is the most popular type of NAT configuration. Overloading is a form of dynamic NAT that maps multiple unregistered IP addresses to a single registered IP address (many-to-one) by using different ports. Therefore, it’s also known as Port Address Translation (PAT). By using PAT (NAT Overload), you can have thousands of users connect to the Internet using only one real global IP address! NAT Overload is the reason we have not run out of valid IP address on the Internet.

Here are shown the logic diagram for NAT point of view and some of the terminology.

Most typical NAT features change only the IP address of “inside” hosts. The outside host IP address can also be changed with NAT. When that occurs, the terms outside local and outside global are used to denote the IP address used to represent that host in the inside network and the outside network, respectively.

This video explains how NAT is used in public and private networks before we discuss IPv6 addresses.

The IPv6 Address

Finally, as you’ve learned by now, we are running out of IP version 4 addresses.  Classless interdomain routing, or CIDR, and Network Address Translation, or NAT, and subnetting subnets have attempted to “address” this issue. IP version 6 is a new addressing scheme to provide an almost unlimited number of addresses.

Issues with IPv4 Addressing

One of things that helped us so much in IP version 4 is almost completely gone in IPv6 and that is Network Address Translation. You may not have been around for the beginnings of proxy servers and NAT, but admins with all of the different applications had to get used to working through NAT, and the technology has caught up with it. Nevertheless, we’re trying to get away from this idea of private and public networks. Instead, we look for global IP addresses, which are equivalent really to public IP addresses and the only local addresses that we have are self-assigned addresses. At least, these are the ones we like to consider private, these local addresses. And so, we’re moving to much more global network where everybody who connects up to it, is uniquely identifiable and the global network and NAT is extracted here so that we have end-to-end reachability without some intermediary reworking IP addresses on the fly.

What is IPv6? Features

All right, so first and foremost with IPv6 – we have many, many more addresses. How many? 3.4 x 10 38 addresses. But let’s put it in perspective, an IPv4 was how many bits? Just 32 bits. IP version 6 addresses are 128 bits. That’s a significant increase in size.

It’s a quadrupling of the bit size, but it grows by 2 to the 96th power, beyond what IPv4 brings to the table. They are 96 bits apart, 2 to the 96, that’s a big number, right? And that larger address base affords us a reason to move to this, namely, scalability. But we don’t want to move over to this technology merely because we want a larger address base. That might be the driving factor, but we want some other benefits along with it. It is very flexible. It allows for route summation. It allows us to connect up to networks and have IP addresses in multiple networks. It also allows more plug-and-play features, self-assigned addresses, and renumbering for switching service providers, all on the fly. A simpler header to make it easier for routers and the core of networks to be able to forward millions, billions, trillions of packets. Security is important. Internet Protocol Security , or IPsec is built into this. If you run IPv6 in some way, you must be capable of working with IPsec.

The thing is, IPv4 is not going away anytime soon. Therefore, we’re going to have a hodgepodge, the old living with the new. That means we’re going to lean on strategies that allow us to run both.

IPv6 addresses

At this point, are you able to pick an IP version address out of a lineup? If we provided you a list, would you know what is valid, what is not valid? That is an important skill to have when it comes to the real world as well. Let’s talk about our IPv6 format. We already said it is 128 bits long, but we don’t break it up in dotted decimal notation, not at all.

Here are some examples:

• 2031:0000:130F:0000:0000:09C0:876A:130B
• FF01:0000:0000:0000:0000:0000:0000:0001
• 0000:0000:0000:0000:0000:0000:0000:0001
• 0000:0000:0000:0000:0000:0000:0000:0000

You see, our IP version 6 addresses utilize hexadecimals, so we have values from 0 through 9 and then A, B, C, D, E, and F that we can find. And we group together four hexadecimal values and then put a colon, and we will do this eight times. So we have eight fields that contain four hexadecimal digits. Let’s think about that. How big are each of these fields? Well each hexadecimal value equates to 4 bits, so four times four would be 16 bits. So we have eight 16-bit fields for a total of 128 bits. Look at the examples provided. These are really long addresses. We had enough problems remembering 192.168.1.8. Try remembering these. It is much, much more difficult. But is this how they are always represented? No, they are not. There are mechanisms in place to shrink them down, truncate them, to make them more manageable, to make them easier to read and it is not just for us, our devices do this as well. So let’s talk about some of these mechanisms here. The first one being to drop leading zeros.

• 2031:0:130F:0:0:09C0:876A:130B
• FF01:0:0:0:0:0:0:1
• 0:0:0:0:0:0:0:1
• 0:0:0:0:0:0:0:0

Look inside one of those fields. Anytime a field starts with a 0, you can take that 0 off. If the next value within that field is a 0, you can take that one off as well. You can remove the first three hexadecimal values in a field if they are zeros. You can’t remove all four because we still have to have a value in there. But we can remove up to the first three leading zeros, truncating each of these fields and making it smaller. That’s option number one, drop leading zeros. But, we might have successive fields of two, three, or four zeros still just taking up space. Can we shrink that down as well?

We absolutely can. And this is one of those things where it’s kind of elastic. t’s like a little spring that is compressed and when released, boom, it springs out and reveals a bunch of zeros. Here is the thing though. We don’t want this address to be too wobbly. Two of these springs, it will get all messy. Here is why.

Importantly, the double colon rule rule gives us the means of inserting four zeros, or substituting four zeros bounded by colons. The address must be bound by colons, whether that includes four, eight, twelve, sixteen, twenty, all the way to thirty two zeros or the entirety of the address

To do this correctly, there are two rules for success. Drop leading zeros, but don’t drop trailing zeros. You might think that some of these examples are a little bit funky. Well they’re actually really good examples. The third example above looks a little weird, doesn’t it? Look at both rules applied to example three. It does not look like the loopback IP address, but that is what it is. So ping ::1, boom, pinged our loopback IP and then certainly we will be talking about routing. And the last entry would be the address portion of a default route. So we would have ::/0, ::/0. The :: representing all zeros IP address /0, a 0 mask. This all comes back full circle!

To summarize, you can only replace one contiguous block of zeros in an address. So if my address has four blocks of zeros and each of them were separated, I just don’t get to replace them all. Remember the rule is that you can only replace one contiguous block with a double colon. Check out this one last example:

2001:0000:0000:0123:0000:0000:4567:89ab Wrong: 2001::0123::4567:89ab Correct: 2001::0123:0:0:4567:89ab

Special Addresses

There is no such thing as a broadcast in IPv6 because it uses multicast traffic instead. There are two other types of communication as well: unicast, which is the same as it is in IPv4, and a new type called anycast. Anycast communication allows the same address to be placed on more than one device so that when traffic is sent to one device addressed in this way, it is routed to the nearest host that shares the same address. In IPv6 we have the following address types:

• Global unicast addresses  – These are your typical publicly routable addresses, and they’re the same as they are in IPv4.
• Link-local addresses  – These are like the private addresses in IPv4 in that they’re not meant to be routed. Think of them as a handy tool that gives you the ability to throw a temporary LAN together for meetings or for creating a small LAN that’s not going to be routed but still needs to share and access files and services locally.
• Unique local addresses – These addresses are also intended for non-routing purposes, but they are nearly globally unique, so it’s unlikely you’ll ever have one of them overlap. Unique local addresses were designed to replace site-local addresses, so they basically do almost exactly what IPv4 private addresses do by allowing communication throughout a site while being routable to multiple local networks. Site-local addresses were denounced as of September 2004.
• Multicast  – Again, same as in IPv4, packets addressed to a multicast address are delivered to all interfaces identified by the multicast address. Sometimes people call them one-to-many addresses. It’s really easy to spot a multicast address in IPv6 because they always start with FF.
• Anycast – Like multicast addresses, an anycast address identifies multiple interfaces, but there’s a big difference: the anycast packet is only delivered to one address-actually, to the first one it finds defined in terms of routing distance. And again, this address is special because you can apply a single address to more than one interface. You could call them one-to-one-of-many addresses, but just saying “anycast” is a lot easier.

In addition, there are a lot of special reserved addresses. We have listed some of them below:

• 0:0:0:0:0:0:0:0 – Equals ::. This is the equivalent of IPv4’s 0.0.0.0, and is typically the source address of a host when you’re using stateful configuration.
• 0:0:0:0:0:0:0:1 – Equals ::1. The equivalent of 127.0.0.1 in IPv4.
• 0:0:0:0:0:0:192.168.100.1 – This is how an IPv4 address would be written in a mixed IPv6/IPv4 network environment.
• 2000::/3 – The global unicast address range.
• FC00::/7 – The unique local unicast range.
• FE80::/10 – The link-local unicast range.
• FF00::/8 – The multicast range.
• 3FFF:FFFF::/32 – Reserved for examples and documentation.
• 2001:0DB8::/32 – Also reserved for examples and documentation.
• 2002::/16 – Used with 6to4, which is the transition system-the structure that allows IPv6 packets to be transmitted over an IPv4 network without the need to configure explicit tunnels.

The sections above are adapted from  “Computer Networking : Principles, Protocols and Practice, 3rd Edition”  by  Olivier Bonaventure  (2019),  Université catholique de Louvain (UCL) , licensed under  CC BY-NC-SA 4.0  as a derivative from  this page  of the  original work

Network Design

Now think about your own networking environment again. Do you think security could be improved by segmenting the network? Watch this CBT Nuggets video to think about a subnetting design for your network [9:07]!

A unit of data with a payload and header used to transfer data over a network. The header contains information such as the protocol type, source and destination addresses, and packet length. The payload is the actual data being transmitted. Datagrams are an efficient way of transmitting large data over a network as they can be broken down into smaller pieces that are transmitted individually. Some errors that can occur with datagrams include the data arriving out of order or the data failing to reach their intended destination.

Datagram Definition - Glossary | NordVPN. 29 Mar. 2023,  https://nordvpn.com/cybersecurity/glossary/datagram/ . Accessed 11 Sept 2023.

A 32-bit address that is used to identify and create smaller networks in a larger network. A subnet defines a range of IP addresses and routes them into defined network. For example, the mask 255.255.255.0 is a class C network and can have a maximum of 254 addresses.

"What Is a Subnet Mask? Definition, Working, and Benefits", Kanade, Vijay. https://www.spiceworks.com/tech/networking/articles/what-is-subnet-mask/ . [Accessed September 20,2023].

Telecommunications and Networking Copyright © by Rita Mitra; Glenn Brown; Melanie Huffman; and Hongyi Zhu is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License , except where otherwise noted.

Share This Book

• Engineering Mathematics
• Discrete Mathematics
• Operating System
• Computer Networks
• Digital Logic and Design
• C Programming
• Data Structures
• Theory of Computation
• Compiler Design
• Computer Org and Architecture
• Computer Network Tutorial

Basics of Computer Network

• Basics of Computer Networking
• Introduction to basic Networking Terminology
• Goals of Networks
• Basic characteristics of Computer Networks
• Challenges of Computer Network
• Physical Components of Computer Network

Network Hardware and Software

• Types of Computer Networks
• LAN Full Form
• How to Set Up a LAN Network?
• MAN Full Form in Computer Networking
• MAN Full Form
• WAN Full Form
• Introduction of Internetworking
• Difference between Internet, Intranet and Extranet
• Protocol Hierarchies in Computer Network
• Network Devices (Hub, Repeater, Bridge, Switch, Router, Gateways and Brouter)
• Introduction of a Router
• Introduction of Gateways
• What is a network switch, and how does it work?

Network Topology

• Types of Network Topology
• Difference between Physical and Logical Topology
• What is OSI Model? - Layers of OSI Model
• Physical Layer in OSI Model
• Data Link Layer

Session Layer in OSI model

• Presentation Layer in OSI model
• Application Layer in OSI Model
• Protocol and Standard in Computer Networks
• Examples of Data Link Layer Protocols
• TCP/IP Model
• TCP/IP Ports and Its Applications
• What is Transmission Control Protocol (TCP)?
• TCP 3-Way Handshake Process
• Services and Segment structure in TCP
• TCP Connection Establishment
• TCP Connection Termination
• Fast Recovery Technique For Loss Recovery in TCP
• Difference Between OSI Model and TCP/IP Model

Medium Access Control

• MAC Full Form
• Channel Allocation Problem in Computer Network
• Multiple Access Protocols in Computer Network
• Carrier Sense Multiple Access (CSMA)
• Collision Detection in CSMA/CD
• Controlled Access Protocols in Computer Network

SLIDING WINDOW PROTOCOLS

• Stop and Wait ARQ
• Sliding Window Protocol | Set 3 (Selective Repeat)
• Piggybacking in Computer Networks

IP Addressing

• What is IPv4?
• What is IPv6?
• Introduction of Classful IP Addressing
• Classless Addressing in IP Addressing
• Classful Vs Classless Addressing
• Classless Inter Domain Routing (CIDR)
• Supernetting in Network Layer
• Introduction To Subnetting
• Difference between Subnetting and Supernetting
• Types of Routing
• Difference between Static and Dynamic Routing
• Unicast Routing - Link State Routing
• Distance Vector Routing (DVR) Protocol
• Fixed and Flooding Routing algorithms
• Introduction of Firewall in Computer Network

Congestion Control Algorithms

• Congestion Control in Computer Networks
• Congestion Control techniques in Computer Networks
• Computer Network | Leaky bucket algorithm
• TCP Congestion Control

Network Switching

• Circuit Switching in Computer Network
• Message switching techniques
• Packet Switching and Delays in Computer Network
• Differences Between Virtual Circuits and Datagram Networks

Application Layer:DNS

• Domain Name System (DNS) in Application Layer
• Details on DNS
• Introduction to Electronic Mail
• E-Mail Format
• World Wide Web (WWW)
• HTTP Full Form
• Streaming Stored Video
• What is a Content Distribution Network and how does it work?

CN Interview Quetions

• Top 50 Networking Interview Questions (2024)
• Top 50 TCP/IP interview questions and answers
• Top 50 IP addressing interview questions and answers
• Last Minute Notes - Computer Networks
• Computer Network - Cheat Sheet
• Network Layer
• Transport Layer
• Application Layer

Prerequisite : OSI Layer

Introduction : The Session Layer is the 5th layer in the Open System Interconnection (OSI) model. This layer allows users on different machines to establish active communications sessions between them. It is responsible for establishing, maintaining, synchronizing, terminating sessions between end-user applications. In Session Layer, streams of data are received and further marked, which is then resynchronized properly, so that the ends of the messages are not cut initially and further data loss is avoided. This layer basically establishes a connection between the session entities. This layer handles and manipulates data which it receives from the Session Layer as well as from the Presentation Layer.

Working of Session Layer : Session Layer, which is the 5th layer in the OSI model, uses the services provided by The transport layer, enables applications to establish and maintain sessions and to synchronize the sessions.  Now, in order to establish a session connection, several things should be followed.

First thing is we should map the session address to the shipping address. The second thing is that we need to select the required transport quality of service (also referred as QoS) parameters. Next thing is we need to take care of the negotiations which should happen between session parameters. Then we further need to transmit limited transparent user data. Then at last, we need to monitor Data Transfer phase properly. The ability to send larger amount of data files is extremely important and a necessary thing too.

Functions of Session Layer : The session layer being the fifth layer in the OSI model performs several different as well as important functions which are need for establishing as well as maintaining a safe and secure connection.

Following are some of the functions which are performed by Session Layer –

• Session Layer works as a dialog controller through which it allows systems to communicate in either half-duplex mode or full duplex mode of communication.
• This layer is also responsible for token management, through which it prevents two users to simultaneously access or attempting the same critical operation.
• This layer allows synchronization by allowing the process of adding checkpoints, which are considered as synchronization points to the streams of data.
• This layer is also responsible for session checkpointing and recovery.
• This layer basically provides a mechanism of opening, closing and managing a session between the end-user application processes.
• The services offered by Session Layer are generally implemented in application environments using remote procedure calls (RPCs).
• The Session Layer is also responsible for synchronizing information from different sources.
• This layer also controls single or multiple connections for each-end user application and directly communicates with both Presentation and transport layers.
• Session Layer creates procedures for checkpointing followed by adjournment, restart and termination.
• Session Layer uses checkpoints to enable communication sessions which are to be resumed from that particular checkpoint at which communication failure has occurred.
• The session Layer is responsible for fetching or receiving data information from its previous layer (transport layer) and further sends data to the layer after it (presentation layer).

Session Layer Protocols : Session Layer uses some protocols which are required for safe, secure and accurate communication which exists between two-ender user applications. Following are some of the protocols provided or used by the Session Layer –

• AppleTalk Data Stream Protocol (ADSP): ADSP is that type of protocol which was developed by Apple Inc. and it includes a number of features that allow local area networks to be connected with no prior setup. This protocol was released in 1985.  This protocol rigorously followed the OSI model of protocol layering. ADSP itself has two protocols named: AppleTalk Address Resolution Protocol (AARP) and Name Binding Protocol (NBP), both aimed at making system self-configuring.
• Real-time Transport Control Protocol (RTCP): RTCP is a protocol which provides out-of-band statistics and control information for an RTP (Real-time Transport Protocol) session. RTCP’s primary function is to provide feedback on the quality of service (QoS) in media distribution by periodically sending statistical information such as transmitted octet and packet counts or packet loss to the participants in the streaming multimedia session.
• Point-to-Point Tunneling Protocol (PPTP): PPTP is a protocol which provides a method for implementing virtual private networks. PPTP uses a TCP control channel and a Generic Routing Encapsulation tunnel to encapsulate PPP (Point-to-Point Protocol) packets This protocol provides security levels and remote access levels comparable with typical VPN (Virtual Private Network) products.
• Password Authentication Protocol (PAP): Password Authentication Protocol is a password-based authentication protocol used by Point to Point Protocol (PPP) to validate users. Almost all network operating systems, remote servers support PAP. PAP authentication is done at the time of the initial link establishment and verifies the identity of the client using a two-way handshake (Client-sends data and server in return sends Authentication-ACK (Acknowledgement) after the data sent by client is verified completely).
• Remote Procedure Call Protocol (RPCP): Remote Procedure Call Protocol (RPCP) is a protocol that is used when a computer program causes a procedure (or a sub-routine) to execute in a different address space without the programmer explicitly coding the details for the remote interaction. This is basically the form of client-server interaction, typically implemented via a request-response message-passing system.
• Sockets Direct Protocol (SDP): Sockets Direct Protocol (SDP) is a protocol that supports streams of sockets over Remote Direct Memory Access (RDMA) network fabrics. The purpose of SDP is to provide an RDMA-accelerated alternative to the TCP protocol. The primary goal is to perform one particular thing in such a manner which is transparent to the application.

Please Login to comment...

Similar reads.

Improve your Coding Skills with Practice

What kind of Experience do you want to share?