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