Used with Ethernet to discover IP Addresses (How Data Link Layer and Network Layer communicate) The IP Stack needs to send to an IP address A on the same ethernet, but doesn’t know what destination MAC Address to use Broadcast an ARP-request message to the ethernet Network: “Who has IP address A?” Wait on Unicast ARP-reply: “I have IP address A” Store response in ARP Table, discarding entries as they become stale Use ARP Table to set ethernet frame’s destination MAC address