Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Part Two
The Protocol Suite of TCP/IP

Chapter 61
Address Resolution Protocol (ARP)

The Internet, but not the TCP/IP protocol, grew up with high-speed local networks such as Ethernet, Token Ring, and FDDI. Before the Internet, there was the ARPAnet and this too ran the TCP/IP protocol. The ARPAnet started on serial lines to communicate between the sites and Ethernet, or any LAN for that matter, was not a consideration. IP addressing worked just fine in this environment. Routing was accomplished between message processors known as IMPs (Interface Message Processors). The hosts connected to the IMP and the IMP connected to the phone lines, which interconnected all ARPAnet sites. The IP address identified the host (and later the network and subnetwork). There was not a need for physically identifying a host for there was only one host per physical connection to the IMP. Multiple hosts could connect to an IMP, but each had an IP address to which the IMP forwarded the information.

Ethernet was commercially available in 1980 and started to gain more recognition when version 2.0 was released in 1982. Since multiple stations were to connect to a network (single cable segment) like Ethernet, each station had to be physically identified on the Ethernet. The designers of Local Area Networks (LANs) allotted 48 bits to identify a network attachment. This is known as a physical address or MAC address. Physical addresses identify stations at their datalink level. IP is an addressing scheme used at the network level. On a LAN (Ethernet, Token Ring, etc.), two communicating stations can set up a session

only if they know each other’s physical address. Think of a MAC address as the number on your house. Lots of houses on your street and each uniquely identified by the number. This is a MAC address.

Since the MAC address is 48 bits and IP is 32 bits, a problem existed and an RFC resolved this problem. The resolution was simple and it did not affect the already established IP addressing scheme. It is known as Address Resolution Protocol, or ARP. This is an IP-address-to-physical-station-address resolution (actual name is binding).

If you are trying to communicate to a host on the same network number as the one on which you are currently residing, the TCP/IP protocol will use ARP to find the physical address of the destination station. If the network number of the destination station is remote, a router must be used to forward the datagram to the destination. The ARP process is used here as well, but only to find the physical address of the router.

There have been enhancements to this protocol although not through an RFC. Some stations listen to all ARP packets since the originator sends them in broadcast mode. All stations receive these packets and will glean the information that they need. The information in the packets includes the senders’ hardware and IP address mapping. In some instances, this information is used by other stations to build their ARP cache. Many ARP tables (cache) empty their tables periodically to reduce the cycles needed to refresh the cache, to conserve memory, and to keep the table up to date. If a station moves from one subnet to another and stations on the subnet do not empty their tables, they will continue to have an entry for that hardware address. ARP is defined in RFC 826.