In the internet in Figure 9.5, we have three links and two routers. We also have
shown only two hosts: Alice (source) and Bob (destination). For each host, we have
shown two addresses, the IP addresses (N) and the link-layer addresses (L). Note
that a router has as many pairs of addresses as the number of links the router is connected to. We have shown three frames, one in each link. Each frame carries the
same datagram with the same source and destination addresses (N! and N8), but the
link-layer addresses of the frame change from link to link. In link 1, the link-layer
addresses are Ll and L2. In link 2, they are L4 and L5. In link 3, they are L7 and Ls.
Note that the IP addresses and the link-layer addresses are not in the same order. For
IP addresses, the source address comes before the destination address; for link-layer
addresses, the destination address comes before the source. The datagrams and frames are designed in this way, and we follow the design. We may raise several
questions: