When a system with a local disk is bootstrapped it normally obtains its IP address from a configuration file that's read from a disk file. But a system without a disk, such as an X terminal or a diskless workstation, needs some other way to obtain its IP address.
Each system on a network has a unique hardware address, assigned by the manufacturer of the network interface. The principle of RARP is for the diskless system to read its unique hardware address from the interface card and send an RARP request (a broadcast frame on the network) asking for someone to reply with the diskless system's IP address (in an RARP reply).
While the concept is simple, the implementation is often harder than ARP for reasons described later in this chapter. The official specification of RARP is RFC 903 Finlayson et al. 1984].