Address Resolution Protocol (ARP) translates IP addresses to Ethernet hardware addresses. Each Ethernet network interface has a unique hardware address. Address resolution maps a host IP address to its unique network interface hardware address. An ARP request is sent to the network; the node that has the IP address returns its hardware address.
Note ARP is used to refer to the process of finding the hardware address and its opposite, Reverse Address Resolution Protocol (RARP). RARP finds the IP address using the hardware address. ARP is transparent to Winsock applications, but when address resolution fails, it usually causes a Winsock function error.