Address Resolution Protocol (ARP) and Reverse ARP (RARP)

Address Resolution Protocol (ARP) 
The Network layer protocol, ARP, associates the physical hardware address of a network node to its already known IP address. Using ARP, an IP process constructs a table (known as the ARP cache) that maps logical addresses to the hardware addresses of nodes on the local network. When a node needs to send a packet to a known IP address on the local subnet, it first checks the ARP cache to see if the physical address information is already present. If so, that address is used and network traffic is reduced; otherwise, a normal ARP request is made to determine the address.

Note:

See Chapter 4, “TCP/IP Utilities,” for more on ARP.

 Reverse ARP (RARP)
 is nothing more than ARP packets with different codes in the header, indicating to devices receiving RARP packets that these are requests by the source device for its own IP configuration, meaning RARP replies should be handled by a RARP server and that any device not fulfilling this role need not process these requests any further. If, however, the receiving device is a RARP server, it is incumbent upon that device to find the requesting device’s MAC address in a configured list (RARP is an older, manual process, unlike DHCP). The server sends the IP address it finds associated with the requesting MAC address back to the requesting device. RARP was adequate for diskless workstation initial IP configuration but fell short as an be-all, end-all supplier of detailed IP-related information, which is why DHCP has supplanted
RARP for supplying network-based IP configuration in most modern networks.