In computer network using the internet protocol suite, the Address Resolution Protocol is a method for finding a host s Ethernet (MAC address) address from its IP address. The sender broadcasts an ARP packet containing the Internet address of another host and waits for it (or some other host) to send back its Ethernet address. Each host maintains a cache of address translations to reduce delay and loading. ARP allows the Internet address to be independent of the Ethernet address but it works only if all hosts support it.

ARP is defined in [http://www.ietf.org/rfc/rfc826.txt RFC 826].

The alternative for hosts that do not do ARP is to use a pre-configured mapping of IP addresses to MAC addresses.

= Variants of the ARP protocol =

ARP was not originally designed as an IP-only protocol, even though it is in practice used almost exclusively to resolve IP addresses to MAC addresses.

ARP can be used to resolve MAC addresses for many different Layer 3 protocols. ARP has also been adapted to resolve other kinds of Layer 2 addresses; for example, ATMARP is used to resolve Asynchronous Transfer Mode NSAP addresses in the Classical IP over ATM protocol.

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