The TCP/IP Model

The Transmission Control Protocol/Internet Protocol (TCP/IP) Model is similar to the OSI Reference Model. It was originally developed by the U.S. Department of Defense and actually preceded the OSI model. However, the TCP/IP model is not as widely used as the OSI model today. The most notable difference between the TCP/IP model and the OSI model is that the TCP/IP model consists of only four layers, rather than seven (see Figure 5-9):

Application Layer: Consists of network applications and processes, and loosely corresponds to the upper layers of the OSI model (Application, Presentation, and Session layers)

Transport Layer: Provides end-to-end delivery and corresponds to the OSI Transport Layer

Internet Layer: Defines the IP datagram and routing, and corresponds to the OSI Network Layer

Network Access (or Link) Layer: Contains routines for accessing physical networks and corresponds to the OSI Data Link and Physical layers