Address Resolution Protocol

A Layer 2 protocol that enables hosts to map IP addresses to MAC addresses in their local network.

Since all devices on a network are identified by their IP addresses, but NICs use MAC addresses instead, hosts need a way to associate their neighbors in the local network with both their IP and MAC addresses.

Discovery

When Host A wants to communicate with Host B for the first time, it must first broadcast an ARP packet, asking all hosts in the local network "what is the IP address associated with MAC address x?" Then Host B would respond to the ARP broadcast with it's own IP address. From there, the two hosts can now send packets to each other.

Note, IPv6 does NOT use ARP to find network neighbors.