The ARP client and server processes operate on all computers using IP over
the Internet. The processes are normally implemented as part of the software
driver which drives the NIC.
An end-station will construct an Address
Resolution Table when it is connected to a network and attempting to
communicate with devices on its LAN segment. To reduce the number of address
resolution requests, a client normally caches resolved addresses for a (short)
period of time. The ARP cache is of a finite size, and would become full of
incomplete and obsolete entries for computers that are not in use if it was
allowed to grow without check. The ARP cache is therefore periodically flushed
of all entries. This deletes unused entries and frees space in the cache. It
also removes any unsuccessful attempts to contact hosts that are not currently
running.
The arp command can be used on an
end-station to verify that the device is actually communicating with other
devices on its LAN segment.
The presence of
an ARP table indicates that the end-station has at least been connected to the
network in the recent past.