10Jun ARP - Address Resolution Protocol

In computer networking, the Address Resolution Protocol is the method for finding a host’s link layer (hardware) address when only its Internet Layer (IP) or some other Network Layer address is known. ARP is defined in RFC 826. It is Internet Standard STD 37. ARP has been implemented in many types of networks; it is not an IP-only or Ethernet-only protocol. It can be used to resolve many different network layer protocol addresses to interface hardware addresses, although, due to the overwhelming prevalence of IPv4 and Ethernet, ARP is primarily used to translate IP addresses to Ethernet MAC addresses. It is also used for IP over other LAN technologies, such as Token Ring, FDDI, or IEEE 802.11, and for IP over ATM. In the next generation Internet Protocol, IPv6, ARP’s functionality is provided by the Neighbor Discovery Protocol (NDP).

ARP is a Link Layer protocol because it only operates on the local area network or point-to-point link that a host is connected to. ARP is also very often discussed in terms of the Open Systems Interconnect (OSI) networking model, because that model addresses hardware-to-software interfaces more explicitly and is preferred by some equipment manufacturers. However, ARP was not developed based on the design principles and strict encapsulation hierarchy of this model and, therefore, such discussions create a number of conflicts as to the exact operating layer within this model. Most often ARP is placed into the Data Link Layer (Layer 2), but it also requires the definitions of network addresses of the Network Layer.

An ARP announcement (also known as Gratuitous ARP) is a packet containing valid sender hardware and protocol addresses (SHA and SPA) for the host that sent it, with identical destination and source addresses (TPA = SPA). Such a request is not intended to solicit a reply, but merely updates the ARP caches of other hosts that receive the packet. Gratuitous ARP is usually an ARP request, but it may also be an ARP reply. Many operating systems perform this during startup. It helps to resolve problems which would otherwise occur if, for example, a network card was recently changed (changing the IP-address-to-MAC-address mapping) and other hosts still have the old mapping in their ARP caches. Gratuitous ARP is also used by some drivers to ensure load balancing on incoming traffic. In a team of network cards, it is used to announce a different MAC address in the team to receive incoming packets.