This illustration uses the image first introduced in figure 1.1 and repeated in subsequent chapters, 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. Each network includes at least one router, though the ISPs and the enterprise network include several. Each router shown here is also associated with a protocol stack; end system H1 and end system H2 have five-layer protocol stacks, while all other protocol stacks have three layers. In all of the three-layer protocol stacks, the top layer is highlighted in blue.

In this image, a laptop computer in the home network is designated as "End system H1." End system H1 is associated with a five-layer protocol stack-from top to bottom, the layers are labeled as follows: Application, Transport, Network, Data link, Physical. End system H1's protocol stack is highlighted in blue at the "Network" layer. The router in the home network is designated as "Router R1," and its three-layer protocol stack is labeled, from top to bottom, as follows: Network, Data link, Physical. The "Network" layer is highlighted in blue. Two of the routers in the Local or Regional ISP are associated with protocol stacks, and both are highlighted in blue at the "Network" layer. Two of the routers in the enterprise network are associated with protocol stacks, and both are highlighted in blue at the "Network" layer. A server in the enterprise network is designated as "End system H2." End system H2 is associated with a five-layer protocol stack and, as with end system H1, the "Network" layer is highlighted in blue.