4.2BSD/usr/man/man4/arp.4p

.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)