NAME

arp − Address Resolution Protocol

SYNOPSIS

pseudo-device ether

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. 

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. 

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. 

SEE ALSO

il(4)

4th Berkeley Distribution  —  1 August 1985