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| 3.6 |  | Implementing Physical Layer Solutions |
| 3.6.2 |  | ARP commands |

The address resolution protocol is used by the IP network layer protocol to map IP network addresses to the hardware addresses used by the data-link protocol. The protocol operates below the network layer as a part of the OSI data-link layer, and is used when IP is used over the Internet.

The MAC address is a data layer address and is dependent on the interface card which is used. IP operates at the network layer and is not concerned with IP addresses of the nodes. A protocol known as the Address Resolution Protocol (ARP) is therefore used to translate between the two types of address. 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 very recent past.