IPv4 – OSI Reference Model

Networking of campuses and communication has always been a need of companies. In the past, every company designs its own network with different specifications and protocols standards. Ancient marketing was based on individual standards 40 years ago. It later proved to be causing difficulties of learning all the standards and odds related to networking efforts. So, a requirement of standardizing the networking method was produced. Different model were presented but layered model for defining the basic standards was the one, most appreciated by the whole world.

Benefits of standard based model and layered approach are:
  • Reduces Complexity
  • Standardizes Interfaces
  • Facilitates  Modular Engineering
  • Ensures Interoperability Technology
  • Acceleration Evaluation
  • Simplifies Learning

The most famous layered model is OSI reference model

OSI (Open System Interconnection) MODEL:

The Open Systems Interconnection (OSI) model (ISO/IEC 7498-1) is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers.

*  Designed by ISO (International Student Organization)

*  Today, every network work on this standard

According to OSI model, network architecture is divided into following seven layers.

                7) – Application Layer

                6) – Presentation Layer

                5) – Session Layer

                4) – Transport Layer

                3) – Network Layer

                2) – Data-Link Layer

                1) – Physical Layer

So OSI is a layered, vender independent model.

OSI model & layered model of detailed description

  • CCNA deals with first three layers of OSI model. We will read them in detail. But first let’s have a brief introduction and general phenomenon in which all these layers work to perform networking purposes.


Let’s take a look at each layer from top to down approach. That means, discussing the layers one by one from top to bottom


The application layer of OSI model is defined as being the user interface. The OSI application layer is responsible for displaying data and images to the user in a human recognizable format and to interface with the presentation layer below it.

  • Whatever is at user-interface, this layer provides it.
  • Some famous Application Layer protocols are:

*  File Transfer Protocols (FTP, TFTP)

*  Electronic Mail Protocols (STMP, IMAP, POP)

*  Support Service Protocols (DNS, RARP, BOOTP, SNMP, CMOT)

*  Remote Login Protocol (telnet)


Presentation Layer serves as the data translator for the Network. It is sometimes called the Syntax Layer.

  • Converts data into appropriate format
  • Extension of the files is a way of representing the file format which is a assigned by Presentation Layer

The Session Layer provides the mechanism for opening, closing and managing a session between end-user application processes.

  • Allocation of all the resource in order to start receiving information, so tat communication can be made effective.
Transport Layer provides logical communication between process running on different hosts. By logical communication we mean that from an application perspective, it is as if the hosts running the processes are directly connected; in reality, the hosts may be on the opposite side of the planet. (Definition by the book: “Computer Networks (A Top-Down Approach)”)

  • Segmentation and Sequencing of data take place
  • Big data is divided into small segments (Segmentation)
  • Assign some number to each chunk of data (Sequencing)
  • TCP (Connection oriented) and UDP (Connectionless) protocols work on this layer


The network layer is responsible for packet forwarding including routing through intermediate routers. It is the most important layer for networking in WAN.

  • Provides address to the data (Source and Destination Address)
  • IP (Inter Protocol) work on this layer (the most familiar protocol in Networking)
  • Routers perform action at this layer


The data link layer is the protocol layer that transfers data between adjacent network nodes in a wide area network or between nodes on the same local area network segment. The data link layer provides the functional and procedural means to transfer data between network entities and might provide the means to detect and possibly correct errors that may occur in the physical layer.

  • This layer defines how data is formatted for transmission and how access to network is controlled
  • Adds Frame check sequence (FCS). Provides error detection in receiver at each link


The physical layer consists of the basic networking hardware transmission technologies of a network. All the transmission lines through which data goes from one place to another are the part of this seventh layer of OSI model 

  • Wires and wireless channels(transmits in binary format)
  • Connectors to connect wires with the network devices
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