Recall that each node on an IP network has both a MAC address and an IP address. In order to send data, the node must use both of these addresses. The node must use its own MAC and IP addresses in the source fields and must provide both a MAC address and an IP address for the destination. While the IP address of the destination will be provided by a higher OSI layer, the sending node needs a way to find the MAC address of the destination for a given Ethernet link. This is the purpose of ARP.

ARP relies on certain types of Ethernet broadcast messages and Ethernet unicast messages, called ARP requests and ARP replies.

The ARP protocol provides two basic functions: