Windows NT TCP/IP Network Administration

Address Resolution

The IP address and the routing table direct a datagram to a specific physical network, but when data travels across a network, it must obey the physical layer protocols used by that network. The physical networks that underlay the TCP/IP network do not understand IP addressing. Physical networks have their own addressing schemes, and there are as many different addressing schemes as there are different types of physical networks. One task of the network access protocols is to map IP addresses to physical network addresses.

The most common example of this network access layer function is the translation of IP addresses to Ethernet addresses. The protocol that performs this function is Address Resolution Protocol (ARP), which is defined in RFC 826.

The ARP software maintains a table of translations between IP addresses and Ethernet addresses. This table is built dynamically. When ARP receives a request to translate an IP address, it checks for the address in its table. If the address is found, it returns the Ethernet address to the requesting software. If the address is not found in the table, ARP broadcasts a packet to every host on the Ethernet. The packet contains the IP address for which an Ethernet address is sought. If a receiving host identifies the IP address as its own, it responds by sending its Ethernet address back to the requesting host. The response is then cached in the ARP table.

The arp command displays the contents of the ARP table. The arp command ...