Address Resolution Protocol
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Address_Resolution_Protocol".

In computer networking, the Address Resolution Protocol (ARP) is the method for finding a host's hardware address when only its network layer address is known. ARP is defined in RFC 826. It is a current Internet Standard, STD 37.

ARP is not an IP-only or Ethernet-only protocol; it can be used to resolve many different network-layer protocol addresses to 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.

For IPv6 ARP's functionality is provided by the Neighbor Discovery Protocol (NDP).

Examples

ARP is used in four cases of two hosts communicating:

1. When two hosts are on the same network and one desires to send a packet to the other
2. When two hosts are on different networks and must use a gateway/router to reach the other host
3. When a router needs to forward a packet for one host through another router
4. When a router needs to forward a packet from one host to the destination host on the same network

The first case is used when two hosts are on the same physical network (that is, they can directly communicate without going through a router). The last three cases are the most used over the Internet as two computers on the Internet are typically separated by more than 3 hops.

In the first case, you would have two hosts, A and B, on the same LAN segment. If, for example, Host A wants to send an IPv4 packet to Host B, Host A must already have an IPv4 (network layer) address for Host B. However, in order to be able to send the packet on the LAN to Host B, Host A must also have a Link Layer address, i.e. a MAC address, for Host B. If it doesn't already know that MAC address, it would send an ARP request to ask for that MAC address, in hope of getting a reply from Host B, or another host on the network, returning the required MAC address.

In the second case, for the same example, hosts A and B would be on different network segments, but there would be a router on the same LAN segment as Host A, which is either on the same network segment as Host B, or on the same network segment as another router that is on the same network segment as Host B, or on the same network segment as another router that is on the same network segment as yet another router that is on the same segment as Host B, and so on. Host A would send the IPv4 packet not to Host B, but to the first of those routers; it would look up Host B in its routing table to determine the IPv4 address of the appropriate router. It would then, if it doesn't already know the MAC address of that router, use ARP to determine that MAC address.

The third case is similar to the second case; the router would look up Host B in its routing table to determine the IPv4 address of the next router to which it should send the packet and, if it doesn't already know the MAC address for the router, use ARP to determine that MAC address. The fourth case is similar to the first case; the router has determined that Host B is on the same LAN segment, and, if it doesn't already know Host B's MAC address, will use ARP to determine that MAC address.

Variants of the protocol

ARP has also been adapted to resolve other kinds of Layer 2 addresses; for example, ATMARP is used to resolve ATM NSAP addresses in the Classical IP over ATM protocol.

ARP mediation

ARP mediation refers to the process of resolving Layer 2 addresses when different resolution protocols are used on either circuit, e.g. ATM on one end and Ethernet on the other.

Inverse ARP and Reverse ARP

The Inverse Address Resolution Protocol, also known as Inverse ARP or InARP, is a protocol used for obtaining Layer 3 addresses (e.g. IP addresses) of other stations from Layer 2 addresses (e.g. the DLCI in Frame Relay networks). It is primarily used in Frame Relay and ATM networks, where Layer 2 addresses of virtual circuits are sometimes obtained from Layer 2 signaling, and the corresponding Layer 3 addresses must be available before these virtual circuits can be used.

ARP translates Layer 3 addresses to Layer 2 addresses, therefore InARP can be viewed as its inverse. In addition, InARP is actually implemented as an extension to ARP. The packet formats are the same; only the operation code and the filled fields differ.

Reverse ARP (RARP), like InARP, also translates Layer 2 addresses to Layer 3 addresses. However, RARP is used to obtain the Layer 3 address of the requesting station itself, while in InARP the requesting station already knows its own Layer 2 and Layer 3 addresses, and it is querying the Layer 3 address of another station. RARP has since been abandoned in favor of BOOTP which was subsequently replaced by DHCP.

Packet structure

The following is the packet structure used for ARP requests and replies. On Ethernet networks, these packets use an EtherType of 0x0806, and are sent to the broadcast MAC address of FF:FF:FF:FF:FF:FF. Note that the packet structure shown in the table has SHA, SPA, THA, & TPA as 32-bit words but this is just for convenience — their actual lengths are determined by the hardware & protocol length fields.

Hardware type (HTYPE) 

Each data link layer protocol is assigned a number used in this field. For example, Ethernet is 1.

Protocol type (PTYPE) 

Each protocol is assigned a number used in this field. For example, IPv4 is 0x0800.

Hardware length (HLEN) 

Length in bytes of a hardware address. Ethernet addresses are 6 bytes long.

Protocol length (PLEN) 

Length in bytes of a logical address. IPv4 address are 4 bytes long.

Operation 

Specifies the operation the sender is performing: 1 for request, and 2 for reply.

Sender hardware address (SHA) 

Hardware address of the sender.

Sender protocol address (SPA) 

Protocol address of the sender.

Target hardware address (THA) 

Hardware address of the intended receiver. This field is ignored in requests.

Target protocol address (TPA) 

Protocol address of the intended receiver.

Example request

If a host with IPv4 address of 10.10.10.123 (0A.0A.0A.7B in hexadecimal notation) and MAC address of 00:09:58:D8:11:22 wants to send a packet to another host at 10.10.10.140 (0A.0A.0A.8C in hexadecimal notation) but it does not know the MAC address, then it must send an ARP request to discover the address. The packet shown shows what would be broadcast over the local network.

Example reply

Given the scenario laid out in the request section, if the host 10.10.10.140 is running and available, then it would receive the ARP request and send a reply packet as shown below. (This reply assumes that the host 10.10.10.140 has a MAC address of 00:09:58:D8:33:AA.) Note that the sender and target address blocks have been swapped (the sender of the reply is the target of the request; the target of the reply is the sender of the request). Furthermore, the host 10.10.10.140 has filled in its MAC address in the sender hardware address.

Any hosts on the same network as these two hosts would also see the request (since it is a broadcast) so they are able to cache information about the source of the request. The ARP reply (if any) is directed only to the originator of the request so information in the ARP reply is not available to other hosts on the same network.

ARP Announcements

An ARP announcement (also known as "Gratuitous ARP") is a packet (usually an ARP Request ^[1]) containing a valid SHA and SPA for the host which sent it, with TPA equal to SPA. Such a request is not intended to solicit a reply, but merely updates the ARP caches of other hosts which receive the packet.

This is commonly done by many operating systems on startup, and helps to resolve problems which would otherwise occur if, for example, a network card had recently been changed (changing the IP-address-to-MAC-address mapping) and other hosts still had the old mapping in their ARP caches.

ARP announcements are also used to defend link-local IP addresses in the (Zeroconf) protocol (RFC 3927), and for IP address takeover within high-availability clusters.

ARP Probe

This term is used in the "IPv4 Address Conflict Detection" specification (RFC 5227). Before beginning to use an IPv4 address (whether received from manual configuration, DHCP, or some other means), a host implementing this specification MUST test to see if the address is already in use, by broadcasting ARP probe packets. An ARP Request constructed with an all-zero 'sender IP address' is referred to as an "ARP Probe".

See also

• Arping
• Arpwatch
• ARP spoofing
• Proxy ARP
• Reverse ARP (RARP)
• Serial line ARP
• Zeroconf

External links

• RFC 826 - Address Resolution Protocol, a.k.a. STD 37.
• RFC 903 - Reverse Address Resolution Protocol, a.k.a. STD 38.
• RFC 2390 - Inverse Address Resolution Protocol, draft standard
• draft - IPv4 Address Conflict Detection
• ARP Sequence Diagram (pdf)
• Gratuitous ARP
• ARP Questions and Answers
• Free ARP tools with source code (French)
• ARP-SK ARP traffic generation tools*
• ARP demo

References

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.