Question : What are the four layers of the TCP/IP protocol?
Because UNIX systems almost always operate in a networked environment, and because UNIX networking is synonymous with TCP/IP, a UNIX administrator must have a general understanding of the principles of TCP/IP networking. Therefore, we will begin our discussion of UNIX networking with some general remarks on TCP/IP.
A network protocol, such as TCP/IP, is a systematic method of enabling processes on one computer to communicate with processes on other computers. To make this rather formidable task approachable, network engineers divide the overall problem into a set of subproblems. They devise systems to solve these subproblems, and then glue the solutions together to solve the overall problem. The buzz word for these subproblems is “layers.” Each layer in a networking protocol solves one stage of the overall problem of moving data between machines.
The layers together form a protocol stack .
TCP/IP divides the problem of moving data into four subproblems, so the TCP/IP protocol stack has four layers.

These layers are: