When an Internet Protocol (IP) datagram is sent from one host to another in a local area network, the destination IP address must be resolved to a MAC address for transmission via the data link layer. When another host's IP address is known, and its MAC address is needed, a broadcast packet is sent out on the local network. This packet is known as an "ARP request". The destination machine with the IP in the ARP request then responds with an "ARP reply" that contains the MAC address for that IP.