The Address Resolution Protocol (ARP), defined in RFC 826 by the Internet Engineering Task Force (IETF), is a fundamental networking protocol used to map Internet Protocol (IP) addresses to their corresponding Ethernet Media Access Control (MAC) addresses. Since IP addresses function at the OSI model's Network Layer (Layer 3), they cannot be directly utilized at the Data Link Layer (Layer 2), which instead relies on MAC addresses for transmitting Ethernet frames.
To establish data communication over Ethernet, the initiating host broadcasts an ARP request to all nodes connected within the same network segment, embedding the target node's IP address within this request. The node possessing the requested IP address responds directly with an ARP reply containing its own MAC address. Upon receiving this reply, the originator can then encapsulate IP packets into Ethernet frames addressed appropriately at Layer 2.
ARP specifically serves IPv4 networks. In IPv6 networks, a functionally equivalent protocol called Neighbor Discovery Protocol (NDP)—as standardized in RFC 4861—manages IP-to-MAC resolution tasks.