arp(5P) arp(5P)NAME arp - Address Resolution Protocol DESCRIPTION arp is a protocol used to dynamically map between DARPA In- ternet and 10Mb/s Ethernet addresses on a local area net- work. It is used by all the 10Mb/s Ethernet interface drivers and is not directly accessible to users. arp caches Internet-Ethernet address mappings. When an in- terface requests a mapping for an address not in the cache, arp queues the message which requires the mapping and broad- casts a message on the associated network requesting the ad- dress mapping. If a response is provided, the new mapping is cached and any pending messages are transmitted. arp it- self is not Internet or Ethernet specific; this implementa- tion, 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. 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. DIAGNOSTICS ``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. April, 1990 1