3.5.3 Address resolution on LANS

A host finds the hardware address that corresponds to an IP address by referring to a look-up table that contains the translation between the two types of address. The look-up table could be maintained manually by a network administrator, but this would be very slow. A far better solution is for the host to maintain the table automatically. Whenever a host has to send a datagram to an unknown IP address it sends a message to all the other hosts on the LAN. The host that recognises the IP address as its own will respond by returning a message with its physical address. The original host will then pass the physical address and datagram to its host-to-network layer for delivery to the destination host. The translation between IP address and physical address is stored in the look-up table so that, should another datagram arrive for delivery to the same destination host, the original host only has to refer to the look-up table. The protocol that specifies the communication between hosts for the translation of addresses is called the address resolution protocol (ARP).