Four-Layer TCP/IP Network Model

In the figure, you see the four layers of the TCP/IP network model on the left. On the right, you see a five-layer model. The one on the left is correct, but many people use the list of five layers on the right. From a practical perspective, it does not matter whether TCP/IP really has four or five layers; you should just know of these layers so that you can talk about TCP/IP.

Each layer represents a general function that must be accomplished. In later chapters, you will learn more about each layer of the TCP/IP model and the protocols that are part of those layers. You can see the protocols mentioned in this chapter in Figure 3-9, next to their related layer. The figure also includes another protocolHypertext Transfer Protocol (HTTP)which web browsers and web servers use.

All networking models are layered, for basically the same reasons. For instance, none of the logic used by TCP for error recovery relies on knowing anything about the physical transmission of the bits. So, the person writing the TCP/IP software on a computer can ignore the physical details. Similarly, a router, like the one in ISP3's network in Figure 3-7, forwards IP packets based on the IP address, but it does not need to be concerned about error recovery. So, layering helps in product development, and it helps in keeping each protocol simple. The reduced complexity makes for better products and more stable networks.

Well, you are now dangerous with regards to what TCP/IP really isa set of protocols and standards, some not even defined directly by TCP/IP, that allow you to create networks. You can find countless books on TCP/IP. Before moving on to the next chapter, however, you should know a few things about Open Systems Interconnect (OSI).