Comparing the OSI and TCP/IP Models

The TCP/IP Model was created by the Defense Advanced Research Projects Agency (DARPA). It has a similar goal as the OSI Model—to provide a common framework for connecting multiple computers together. However, different sources show this model as having either four or five layers. Additionally, different sources show these layers with different names. Figure 3-3 compares the four-layer and five-layer TCP/IP Models with the OSI Model.

Figure 3-3 TCP/IP model compared to the OSI model

TIP The TCP/IP Model is also called the DOD Model (DOD being an acronym for the Department of Defense).

You can see that the upper-three layers of the OSI Model map directly to the application layer of the TCP/IP Model. When five layers are shown, you’ll often see the bottom-four layers named exactly the same as they are on the OSI Model. In essence, they perform the same tasks.

RFCs 1122 and 1123 identify the TCP/IP Model with four layers and provide different names. The OSI transport layer maps to the TCP/IP transport layer, but it is sometimes referred to as the host, or host-to-host, layer. The OSI network layer maps to the Internet layer. Finally, the OSI data link and physical layers map to the TCP/IP link layer, and this layer is sometimes called the network interface or network access layer.

TIP Although RFCs 1122 and 1123 identify the TCP/IP Model with four layers, some references listed in the Candidate Information Bulletin (CIB) identify it with five layers.