In computer networking ARP is a method for finding a host's Ethernet (MAC) 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 only works if all hosts support it.
| Application layer | FTP | SMTP | HTTP | ... |
| Transport layer | TCP | UDP | ||
| Network layer | IP ICMP | ARP | ||
| data link layer | Ethernet | Token Ring | FDDI | ... |
ARP is defined in RFC 826.
The alternative for hosts that do not do ARP is to use a pre-configured mapping of IP addresses to MAC addresses.
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 ATM NSAP addresses in the Classical IP over ATM protocol.
See also proxy ARP, reverse ARP, serial line ARP, Zeroconf
Based on Foldoc used with permission
External links:
Variants of the ARP protocol