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 disk-less 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 (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 disk-less workstation booted up).
Note: RARP uses the same PDU header as ARP. See ARP PDU.
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