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* The address
resolution protocol (ARP) is a protocol used by the Internet
Protocol (IP) specifically IPv4, to map IP network addresses to the hardware
addresses used by a data link protocol.
* The protocol
operates below the network layer as a part of the interface between the OSI
network and OSI link layer. It is used when IPv4 is used over Ethernet.
* It is also
used for IP over other LAN technologies, such as Token Ring, FDDI, or IEEE 802.11,
and for IP over ATM.
* ARP is a Link
Layer protocol because it only operates on the local area network or point-to-point
link that a host is connected to.
* The hardware
address is also known as the Medium Access Control (MAC) address, in reference
to the standards which define Ethernet.
* The Ethernet
address is a link layer address and is dependent on the interface card which is
used.
* IP operates at
the network layer and is not concerned with the link addresses of individual
nodes which are to be used. The ARP is therefore used to translate IP addresses
into MAC address.
* In the below figure suppose host H1
wants to send an IP packet to H3, but does not know the MAC address of H3. H1
first broadcast an ARP request packet asking the destination host, which is
identified by H3’s IP address, to reply. All hosts in the network receive the
packet, but only the intended host, which is H3, responds to H1.
* The ARP response packet contains H3’s
MAC address and IP addresses.
* H1 caches H3’s MAC address in its ARP
table so that H1 can simply look up H3’s MAC address in the table for future
use.
* The ARP client
and server processes operate on all computers using IP over Ethernet. The
processes are normally implemented as part of the software driver that drives the
network interface card.
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