Address Resolution Protocol
The address resolution protocol (arp) is a protocol used by the Internet Protocol (IP), specifically IPv4, to map IP network addresses to the hardware addresses used by a data link protocol.
The protocol operates below the network layer as a part of the interface between the OSI network and OSI link layer. It is used when IPv4 is used over Ethernet.
ARP is described in RFC 826
An Ethernet Address Resolution Protocol -- or -- Converting Network Protocol Addressesvto 48.bit Ethernet Address for Transmission over Ethernet Hardware
Reverse ARP
Reverse ARP, document in RFC 903, is a fairly simple bootstrapping protocol that allows a workstation to broadcast using its Ethernet address, and expect a server to reply, telling it its IP address.