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Address Resolution Protocol (ARP)

Networking Topics > Layer 3 Routing > Address Resolution Protocol (ARP)

Address Resolution Protocol (ARP) is essentially a protocol designed so hosts can retrieve MAC addresses for other hosts they do not know about.

Most hosts on a network (Computers, Servers, Routers, Bridges, Switches etc) build ARP tables, which store MAC addresses of all the devices on the network, and their associated IP addresses. These ARP tables are stored in RAM and are maintained automatically.

There are times however when a host needs to send data to a host and it does not have the MAC address of that host in it's ARP tables. In this situation the host will send an ARP request containing the IP address of the host it wishes to know the address of. On receiving the information it will add an entry into it's ARP table in case it needs to send data to that host again.

ARP requests are sent as broadcast packets which have a MAC address of "FF-FF-FF-FF-FF-FF" and because they are broadcast packets all hosts on the local network segment receive the packet and examine the IP address against their own IP. Only on a match will an ARP reply be sent back to the source of the ARP request.

ARP requests are not forwarded by routers and therefore only remain on the local network segment (Broadcast domain).

If the host that the source is looking for exists outside the local segment the default gateway responds. The default gateway, usually a router, acts as an intermediary and responds to the ARP request with a proxy ARP reply containing it's own MAC address. This will only happen if the IP address is known to exist and it is on a different segment to the local segment.

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