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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Network Interface / Internet "Layer Connection" Protocols            9  Address Resolution and the TCP/IP Address Resolution Protocol (ARP)                 9  TCP/IP Address Resolution Protocol (ARP)

TCP/IP Address Resolution Protocol (ARP) 1 ARP Address Specification and General Operation

Physical networks function at layers one and two of the OSI Reference Model, and use data link layer addresses. In contrast, internetworking protocols function at layer three, interconnecting these physical networks to create a possibly huge internetwork of devices specified using network layer addresses. Address resolution is the process where network layer addresses are resolved into data link layer addresses, to permit data to be sent one hop at a time across an internetwork. I describe address resolution in detail in the preceding section on concepts.

The problem of address resolution was apparent from the very start in the development of the TCP/IP protocol suite. Much of the early development of IP was performed on the then-fledgling Ethernet local area networking technology; this was even before Ethernet had been officially standardized as IEEE 802.3. It was necessary to define a way to map IP addresses to Ethernet addresses to allow communication over Ethernet networks.

There are two basic methods that resolution could have been used to accomplish this correlation of addresses: direct mapping or dynamic resolution. However, Ethernet addresses are 48 bits long while IP addresses are only 32 bits, which immediately rules out direct mapping. Furthermore, the designers of IP wanted the flexibility that results from using the dynamic resolution model. To this end, they developed the TCP/IP Address Resolution Protocol (ARP). This protocol is described in one of the earliest of the Internet RFCs still in common use: RFC 826, An Ethernet Address Resolution Protocol, published in 1982.

The name makes clear that ARP was originally developed for Ethernet. Thus, it represents a nexus between the most popular layer two LAN protocol and the most popular layer three internetworking protocol—this is true even two decades later. However, it was also obvious from the beginning that even if Ethernet was a very common way of transporting IP, it would not be the only one. Therefore, ARP was made a general protocol capable of resolving addresses from IP to not just Ethernet but numerous other data link layer technologies.

The basic operation of ARP involves encoding the IP address of the intended recipient in a broadcast message. It is sent on a local network to allow the intended recipient of an IP datagram to respond to the source with its data link layer address. This is done using a simple request/reply method described in the following topic on general operation. A special format is used for ARP messages, which are passed down to the local data link layer for transmission.

This basic operation is supplemented by methods to improve performance. Since it was known from the start that having to perform a resolution using broadcast for each datagram was ridiculously inefficient, ARP has always used a cache, where it keeps bindings between IP addresses and data link layer addresses on the local network. Over time, various techniques have been developed to improve the methods used for maintaining cache entries. Refinements and additional features, such as support for cross-resolution by pairs of devices as well as proxy ARP, have also been defined over the years and added to the basic ARP feature set.

TCP/IP Address Resolution Protocol (ARP) 1 ARP Address Specification and General Operation