ARP (Address Resolution Protocol)
A protocol for mapping an Internet Protocol (IP) address to a physical computer address that is recognized in the local network. When an interface on one computer needs to talk to another interface, it will ARP (that is, send out a broadcast) asking for a response from the interface that matches the IP address. The response contains the hardware address of the interface that has the corresponding IP address.
A low-level TCP/IP protocol, which is used to obtain the MAC address of a device when its IP address is known.