This illustration uses the image introduced in and used throughout chapter one, beginning in figure 1.1, of the interconnected networks that make up the Internet. These networks include a Mobile Network, a National or Global ISP, a Local or Regional ISP, a Home Network, and an Enterprise Network. This figure includes three protocol stacks that read:

One protocol stack appears next to a laptop in the Mobile Network, and one appears next to a server in the Enterprise Network; these are connected by a blue arrow that points from one to the other, and the word "Application" is highlighted in blue. There is a third protocol stack beside a desktop computer in the Enterprise Network; this is connected to the server by a second blue arrow that points from one back to the other, and the word "Application" is also highlighted in blue.