We covered the OSI Model in CCNA Lesson 5, now we move on to the TCP/IP Model which you will also need to know for the CCNA exam. Since internetworking is based on the TCP/IP protocol suite, the TCP/IP model is a bit more important than the OSI model on the CCNA exam.
While the ISO folks were meeting in committees to develop the OSI Reference Model, the rest of the world got busy making networks actually work and the TCP/IP suite of protocols is the result. The TCP/IP model of networking describes how the TCP/IP protocol suite functions and operates. Simpler than the OSI Reference Model, the TCP/IP model describes the most common stack of network protocols in use today. This network model goes by several names: the DARPA Model, the Department of Defense (DoD) Model, and today we just call it the TCP/IP Model.
There is a full tutorial on the TCP/IP Model here at InetDaemon.com.