ARP is the protocol that maps IP addresses to MAC addresses on local networks.
The Address Resolution Protocol, or ARP, lets an IPv4 host discover the link-layer hardware address associated with a destination IP address. On an Ethernet network, a device broadcasts an ARP request asking which host owns an IP address. The matching host replies with its MAC address, allowing the sender to place the IP packet inside an Ethernet frame addressed to the correct interface.
ARP operates within the local subnet and is not routed across the wider Internet. Devices usually store successful results in an ARP cache so they do not need to broadcast a request for every packet. ARP was defined in 1982 and standardized as Internet Standard STD 37.
The protocol is specific to IPv4. IPv6 uses Neighbor Discovery Protocol instead. DHCP helps configure addresses, DNS maps names to IP addresses, and ICMP carries diagnostic and control messages. ARP also lacks built-in authentication, which is why ARP spoofing can enable local man-in-the-middle attacks.