RARP is a protocol by whcih a physical machine in alocal area network can request to learn its IP address from a gateway server's ARP cache. A network administrator creates a table in alocal area  network's gateway router that maps  the physical machine addresses
(mac address) to corresponding IP address. When a new machine is setup its RARP client program requests from the RARP server on the router to be sent its IP address. Assuming that an entry has been set up in the router table the RARP server will return the IP address to the machine which can store it for future use.