Why Do We Need a Network Model?

Because we are about to spend some of your valuable time discussing a theoretical model, it is only reasonable that we first discuss why we have such a model in the first place and how it can help us.

In simple terms, the OSI model provides a structure that helps us work with networks. By relating services and devices to a certain layer of the model, you can get a better idea of their function and purpose. For example, recall from Chapter 3, “Networking Components and Devices,” that switches use the Media Access Control (MAC) address of the attached devices to make forwarding decisions. In the OSI model, MAC addresses are defined in the MAC sublayer of the data-link layer (Layer 2). If you knew that a bridge was also a data-link-layer device, you could quite reasonably draw the conclusion that it, too, works with MAC addresses—and you would be right. This example is perhaps one of the simplest that we could have used, but it serves the purpose well: It shows how the theoretical model can be translated into actual scenarios.