To show how communication is done at the data-link layer and how link-layer addresses
are found, let us go through a simple example. Assume Alice needs to send a datagram
to Bob, who is three nodes away in the Internet. How Alice finds the network-layer
address of Bob is what we discover in Chapter 26 when we discuss DNS. For the
moment, assume that Alice knows the network-layer (IP) address of Bob. In other
words, Alice's host is given the data to be sent, the IP address of Bob, and the IP address of Alice's host (each host needs to know its IP address). Figure 9.10 shows
the part of the internet for our example.