The Internet is a combination of networks glued together by connecting devices (routers
or switches). If a packet is to travel from a host to another host, it needs to pass
through these networks. Figure 9.1 shows the same scenario we discussed in Chapter 3,
but we are now interested in communication at the data-link layer. Communication at
the data-link layer is made up of five separate logical connections between the data-link
layers in the path.