DoD Network Model or TCP / IP Model

DoD Network Model or TCP / IP Model

Previously, we discussed about OSI Model, the Open Systems Interconnection Model, which divided networks into 7 layers namely - Application Layer, Presentation Layer, Session Layer, Transport Layer, Network Layer, Data-Link Layer and Physical Layer. We also stated that the network communication is defined only by the lower 4 layers. Now, we will see the model designed and adopted by Department of Defense (DoD) to connect their networks, which eventually is popularly referred to or known as TCP / IP Model. Hence forth, will refer it as TCP / IP Model.

The major difference between both the models is the number of stacked layers into which the network communication is divided logically. In OSI Model the layers are 7, but in TCP / IP Model it is only 4 layers. These four layers are Application Layer, Transport Layer, Internet Layer and Network Access Layer.

In TCP / IP Model the application, presentation and session layers of OSI are clubbed together into a single layer and named it as Application Layer. The Layer 4 of OSI Model i.e. Transport Layer is same in TCP / IP Model as well, but Layer 3 of OSI Model i.e. Network Layer is named as Internet Layer. The last layer in TCP / IP Model has clubbed together the Layer 2 and Layer 1 of OSI Model into one single Network Access Layer.

Can be graphically represented as, 

It acts as an interface to the user.

means of end-to-end host communication.

Network discovery, Network to Network communication.

Physical Network communication and Host discovery.

Today, all the networks are run based on the TCP / IP Network Model, but the technical people still refer to OSI Network Model as for designing, implementing and troubleshooting the networks irrespective of its base being TCP / IP Model.

TCP / IP Model is easy to implement, hence, the industry used it and went on developing it. Even then, it lacks the clarity of OSI Model in defining the functionality at each layer. Both the models divide the communication into layers logically.

Why people call it as TCP / IP Model ? TCP and IP are nothing but the two main protocols on which the networks of today are working. TCP at Transport Layer and IP at Internet Layer. TCP means Transmission Control Protocol and IP means Internet Protocol.

Until now we discussed the two network models and the constituent layers of each model. Now, let us shift our focus to each individual layer as per the OSI Model definition for better understanding of the functions that each layer carries out.