Reverse Address Resolution Protocol
Reverse Address Resolution Protocol (RARP) is the reverse of ARP. It is a mechanism to map MAC addresses to IP addresses. It is used mainly by diskless workstations upon boot-up to find out their IP addresses from a BOOTP server. The BOOTP server contains all of the boot-up configuration files that the workstation needs to boot-up.
On NICs (network interface cards) there is an empty DIP socket that is used for holding a Boot PROM. The Boot PROM holds a special software program that tells the workstation that upon powering up, to go and find a BOOTP server. One of the first tasks of the workstation is to find out its IP address. The MAC layer address is burnt into the NIC and is already known.
A RARP broadcast datagram is sent out that asks "Does any BOOTP server know what my IP address is?". The BOOTP server will reply with "Here's the IP address that belongs to your MAC address".
Once the IP address is known, then the rest of the configuration files can be downloaded and the diskless workstation booted up.