.TH ARP 4P "18 July 1983" .UC 4 .SH NAME arp \- Address Resolution Protocol .SH SYNOPSIS .B pseudo-device ether .SH DESCRIPTION ARP is a protocol used to dynamically map between DARPA Internet and 10Mb/s Ethernet addresses on a local area network. It is used by all the 10Mb/s Ethernet interface drivers and is not directly accessible to users. .PP ARP caches Internet-Ethernet address mappings. When an interface requests a mapping for an address not in the cache, ARP queues the message which requires the mapping and broadcasts a message on the associated network requesting the address mapping. If a response is provided, the new mapping is cached and any pending messages are transmitted. ARP itself is not Internet or Ethernet specific; this implementation, however, is. ARP will queue at most one packet while waiting for a mapping request to be responded to; only the most recently ``transmitted'' packet is kept. .PP ARP watches passively for hosts impersonating the local host (i.e. a host which responds to an ARP mapping request for the local host's address) and will, optionally, periodically probe a network looking for impostors. .SH DIAGNOSTICS .B "duplicate IP address!! sent from ethernet address: %x %x %x %x %x %x". ARP has discovered another host on the local network which responds to mapping requests for its own Internet address. .SH SEE ALSO ec(4), il(4)