The Address Resolution Protocol is a layer 2 protocol used to map MAC addresses to IP addresses. All hosts on a network are located by their IP address, but NICs do not have IP addresses, they have MAC addresses. ARP is the protocol used to associate the IP address to a MAC address.

When a host wants to send a packet to another host, say IP address 10.5.5.1, on its local area network (LAN), it first sends out (broadcasts) an ARP packet. The ARP packet contains a simple question: What is the MAC address corresponding to IP address 10.5.5.1? The host that has been configured to use the IP address responds with an ARP packet containing its MAC address.