Mapping Internet Addresses to Physical Addresses

An obvious question is 'What physical address should the sender use to send Internet datagrams to a specific Internet site?'

In some cases a physical address may fit into, say, a Class A Internet address, but more typically an Ethernet address (48 bits) will not fit in the Internet addressing schema.

The Address Resolution Protocol (ARP) is a special protocol designed to map Internet to physical addresses. When a gateway needs to know the physical address for an Internet address of a host known to be on its network, it broadcasts an ARP frame requesting the physical address. The required host replies; the gateway caches the address for future reference.