28.2. Example of an ARP Transaction
Figure 28-2 shows a simple case where one host asks for the L2 address associated with the IP address 10.0.0.4, and the owner of that address replies. The MAC target (the address to resolve) in the request is 0, to indicate that it should be filled in by whoever (probably the owner of the target IP address) replies.
If you look carefully at Figure 28-2, you can see that the sender hardware address is present twice: once in the Ethernet header and once in the ARP payload. They usually match, but not always (see the section "Acting As a Proxy" in Chapter 27).
More-detailed examples appear later in the section "Examples."