The Department of Defense (DOD), a division of the United States government, developed a model that would be used as the developing basis for their own protocol suite known as the Internet protocol suite. A protocol suite indicates a group of protocols that were designed and meant to be used together. This model has four layers compared to the seven layers of the OSI model. The figure below shows the four layers of the DOD model and how it maps out to the OSI model.

The DOD’s Process/Application layer maps out to the Application, Presentation and Session layers of the OSI model. The Host-to-Host layer maps out to the Transport layer and the Internet layer maps out to the Network layer. The graphic above shows how the layers of the DOD model map out to the layers of the OSI model. Since there is a relationship between the layers of each of the models, some of the developed protocols in the Internet suite, at a particular layer, functions much like the equivalent layers of the OSI model. An example would be a protocol like Routing Information Protocol (RIP), which functions at the Internet layer of the DOD. Since the Internet layer of the DOD maps out to the Network layer of the OSI model, RIP would have the same responsibility of route discovery, which is an OSI Network layer responsibility.

The Network Access layer is different. The DOD did not develop any protocols for this layer. The DOD developed protocols for the Process/Application, Host-to-Host, and Internet layers only. The DOD did not develop any protocols for the Network Access layer, because they wanted to create a generic suite of protocols that would function on any vendor’s system. It was the responsibility of the individual vendors to create a set of protocols that would allow the Internet suite to work with their hardware. These vendors created protocols that would function at the Network Access layer. This is a main reason why the Internet protocol suite is used on so many different systems.

This generic solution by the Department of Defense allows the capability of making the Internet as popular as it is today on such a vast number of systems.