Address Resolution Protocol (ARP)
ARP is used to map a 32-bit IP address to a 48-bit ethernet address. As you saw in Chapter 1, “Local Area Networks,” an ethernet frame contains the sending and receiving ethernet addresses. If two systems need to communicate across a network, these ethernet addresses are needed; otherwise the ethernet frame will not be delivered to the correct recipient, very much like what would happen if you sent a letter to someone but left the address blank.
ARP uses the broadcast mechanism to try to find a host's ethernet address. The only information that ARP possesses, at this point, is the IP address (say 192.168.0.1), so it basically sends a request to all stations on the local network, asking, “Who has the IP address ...
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