Communication at the data-link layer is node-to-node. A data unit from one point in the
Internet needs to pass through many networks (LANs and WANs) to reach another
point. Theses LANs and WANs are connected by routers. It is customary to refer to the
two end hosts and the routers as nodes and the networks in between as links. Figure 9.2
is a simple representation of links and nodes when the path of the data unit is only six
nodes.