Internet Core Protocols: The Definitive Guide

The ARP Standard

ARP is defined in RFC 826, which has been republished as STD 37 (ARP is an Internet Standard protocol). However, RFC 826 contained some vagaries which were clarified in RFC 1122 (Host Network Requirements). As such, ARP implementations need to incorporate both RFC 826 and RFC 1122 in order to work reliably and consistently with other implementations.

RFC 826 introduced the concept of an Address Resolution Protocol as a useful way for devices to locate the Ethernet hardware address of another IP host on the same local network. As it turns out, ARP is also useful for many network topologies—not just Ethernet—and has since become incorporated into most of the other network topologies. All LAN media—and many WAN media—now use ARP to locate the hardware addresses of other IP devices on the local network.

When a device needs to send an IP packet to another device on the local network, the IP software will first check to see if it knows the hardware address associated with the destination IP address. If so, then the sender just transmits the data to the destination system, using the protocols and addressing appropriate for whatever network medium is in use by the two devices. However, if the destination system’s hardware address is not known, then the IP software has to locate it before any data can be sent. At this point, IP will call on ARP to locate the hardware address of the destination system.

ARP achieves this task by issuing a low-level broadcast onto the network, ...