The Address Resolution Protocol (ARP) finds the hardware address of a host
from a known IP address. Here’s how it works: When IP has a datagram to
send, it must inform a Network Access protocol, such as Ethernet or Token
Ring, of the destination’s hardware address on the local network.
If IP doesn’t find the destination host’s hardware address in the
ARP cache, it uses ARP to find this information.
As IP’s detective, ARP interrogates the local network by sending out a
broadcast asking the machine with the specified IP address to reply with its
hardware address. In other words, ARP translates the software (IP) address
into a hardware address—for example, the destination machine’s Ethernet
board address—and from it, deduces its whereabouts. This hardware address
is technically referred to as the media access control (MAC) address or physical
address. Figure 3.8 shows how an ARP might look to a local network.
Reverse Address Resolution Protocol (RARP)
When an IP machine happens to be a diskless machine, it has no way of initially
knowing its IP address, but it does know its MAC address. The Reverse
Address Resolution Protocol (RARP) discovers the identity of the IP address
for diskless machines by sending out a packet that includes its MAC address
and a request for the IP address assigned to that MAC address. A designated
machine, called a RARP server, responds with the answer, and the identity
crisis is over. RARP uses the information it does know about the machine’s
MAC address to learn its IP address and complete the machine’s ID portrait.