What is Address Resolution Protocol?

ARP

          (Address Resolution Protocol)

       Address Resolution Protocol (ARP) is a protocol for mapping an Internet protocol  (IP address) in to a physical machine address that is recognized by the local network.  

          When an incoming  packet destined  for a host a machine on particular area network arrives at gateway  asks the ARP program to find a physical host or MAC address that matches the IP address. 

 The ARP program looks in the ARP cache, and if it finds the address, provides it so that packet can be converted to the right packet length and format, and sent to the machine. If no entry is found for the IP address, ARP broadcast a request packet in a  special  format to all the machines on the LAN. If one machine knows that it has that IP address associated with it. ARP  updates the ARP cache for future reference and then sends the packet to the MAC address.

         Since protocol details differ for each type of local area network, there are separate ARP Requests for Comments (RFC) for Ethernet, ATM, Fiber Distributed-Data Interface, HIPPI, and other protocols.

         There is a Reverse ARP (RARP)  for host machine that do not know their IP address. 

RARP enables them to request their IP address from the gateway's ARP cache.

Note:  

         This mapping is a very critical function in the Internet protocol Suite. ARP was defined in  1982
 by RFC 826, which is Internet standard STD 37.

ARP has been implemented with many combinations of network and data link layer technologies ,
 such as IPv4 chaosnet, DECnet and Xerox PARC Universal Packet (PUP) using IEEE 802 standards, FDDI, X.25, Frame Relay and Asynchronous Transfer Mode (ATM). IPv4 over IEEE 802.3 and IEEE 802.11 is the most common usage.