Networking: Protocols: ARP - Address Resolution Protocol

The Address Resolution Protocol (ARP) is a communication protocol used for discovering the link layer address, such as a MAC address, associated with a given network layer address, typically an IPv4 address. This mapping is a critical function in the Internet protocol suite. ARP was defined in 1982 by RFC 826,[1] which is Internet Standard STD 37.

ARP has been implemented with many combinations of network and data link layer technologies, such as IPv4, Chaosnet, DECnet and Xerox PARC Universal Packet (PUP) using IEEE 802 standards, FDDI, X.25, Frame Relay and Asynchronous Transfer Mode (ATM). IPv4 over IEEE 802.3 and IEEE 802.11 is the most common usage.

In Internet Protocol Version 6 (IPv6) networks, the functionality of ARP is provided by the Neighbor Discovery Protocol (NDP).

Operating scope

The Address Resolution Protocol is a request-response protocol whose messages are encapsulated by a link layer protocol. It is communicated within the boundaries of a single network, never routed across internetworking nodes. This property places ARP into the link layer of the Internet protocol suite.[2]

Packet structure

The Address Resolution Protocol uses a simple message format containing one address resolution request or response. The size of the ARP message depends on the upper layer and lower layer address sizes, which are given by the type of networking protocol (usually IPv4) in use and the type of hardware or virtual link layer that the upper layer protocol is running on. The message header specifies these types, as well as the size of addresses of each. The message header is completed with the operation code for request (1) and reply (2). The payload of the packet consists of four addresses, the hardware and protocol address of the sender and receiver hosts.

The principal packet structure of ARP packets is shown in the following table which illustrates the case of IPv4 networks running on Ethernet. In this scenario, the packet has 48-bit fields for the sender hardware address (SHA) and target hardware address (THA), and 32-bit fields for the corresponding sender and target protocol addresses (SPA and TPA). Thus, the ARP packet size in this case is 28 bytes. The EtherType for ARP is 0x0806. (This appears in the Ethernet frame header when the payload is an ARP packet. Not to be confused with PTYPE below, which appears within this encapsulated ARP packet.)

Internet Protocol (IPv4) over Ethernet ARP packet
octet offset 0 1
0 Hardware type (HTYPE)
2 Protocol type (PTYPE)
4 Hardware address length (HLEN) Protocol address length (PLEN)
6 Operation (OPER)
8 Sender hardware address (SHA) (first 2 bytes)
10 (next 2 bytes)
12 (last 2 bytes)
14 Sender protocol address (SPA) (first 2 bytes)
16 (last 2 bytes)
18 Target hardware address (THA) (first 2 bytes)
20 (next 2 bytes)
22 (last 2 bytes)
24 Target protocol address (TPA) (first 2 bytes)
26 (last 2 bytes)
Hardware type (HTYPE)
This field specifies the network protocol type. Example: Ethernet is 1.
Protocol type (PTYPE)
This field specifies the internetwork protocol for which the ARP request is intended. For IPv4, this has the value 0x0800. The permitted PTYPE values share a numbering space with those for EtherType.[3][4][5]
Hardware length (HLEN)
Length (in octets) of a hardware address. Ethernet addresses size is 6.
Protocol length (PLEN)
Length (in octets) of addresses used in the upper layer protocol. (The upper layer protocol specified in PTYPE.) IPv4 address size is 4.
Specifies the operation that the sender is performing: 1 for request, 2 for reply.
Sender hardware address (SHA)
Media address of the sender. In an ARP request this field is used to indicate the address of the host sending the request. In an ARP reply this field is used to indicate the address of the host that the request was looking for. (Not necessarily address of the host replying as in the case of virtual media.) Switches do not pay attention to this field, particularly in learning MAC addresses. The ARP PDU is encapsulated in Ethernet frame, and that is what Layer 2 devices examine.
Sender protocol address (SPA)
Internetwork address of the sender.
Target hardware address (THA)
Media address of the intended receiver. In an ARP request this field is ignored. In an ARP reply this field is used to indicate the address of the host that originated the ARP request.
Target protocol address (TPA)
Internetwork address of the intended receiver.

ARP protocol parameter values have been standardized and are maintained by the Internet Assigned Numbers Authority (IANA).[6]


Two computers in an office (computer 1 and computer 2) are connected to each other in a local area network by Ethernet cables and network switches, with no intervening gateways or routers. Computer 1 has a packet to send to Computer 2. Through DNS, it determines that Computer 2 has the IP address To send the message, it also requires Computer 2's MAC address. First, Computer 1 uses a cached ARP table to look up for any existing records of Computer 2's MAC address (00:eb:24:b2:05:ac). If the MAC address is found, it sends an Ethernet frame with destination address 00:eb:24:b2:05:ac, containing the IP packet onto the link. If the cache did not produce a result for, Computer 1 has to send a broadcast ARP message (destination FF:FF:FF:FF:FF:FF MAC address), which is accepted by all computers, requesting an answer for Computer 2 responds with its MAC and IP addresses. Computer 2 may insert an entry for Computer 1 into its ARP table for future use. Computer 1 caches the response information in its ARP table and can now send the packet.[7]

ARP probe

An ARP probe is an ARP request constructed with an all-zero sender IP address (SPA). The 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.[8]

ARP announcements

ARP may also be used as a simple announcement protocol. This is useful for updating other hosts' mappings of a hardware address when the sender's IP address or MAC address has changed. Such an announcement, also called a gratuitous ARP message, is usually broadcast as an ARP request containing the sender's protocol address (SPA) in the target field (TPA=SPA), with the target hardware address (THA) set to zero. An alternative way is to broadcast an ARP reply with the sender's hardware and protocol addresses (SHA and SPA) duplicated in the target fields (TPA=SPA, THA=SHA).

The gratuitous ARP request message and the gratuitous ARP reply messages are standards-based methods,[9][10] but the "ARP Request" is preferred.[11] Some devices may be configured for the use of either of these two types of GARP.[12]

An ARP announcement is not intended to solicit a reply; instead it updates any cached entries in the ARP tables of other hosts that receive the packet. The operation code may indicate a request or a reply because the ARP standard specifies that the opcode is only processed after the ARP table has been updated from the address fields.[13][14][15]

Many operating systems perform gratuitous ARP during startup. That 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 interface drivers to provide load balancing for incoming traffic. In a team of network cards, it is used to announce a different MAC address within the team that should receive incoming packets.

ARP announcements can be used to defend link-local IP addresses in the Zeroconf protocol (RFC 3927), and for IP address takeover within high-availability clusters.[clarification needed][example needed]

ARP mediation

ARP mediation refers to the process of resolving Layer 2 addresses through a Virtual Private Wire Service (VPWS) when different resolution protocols are used on the connected circuits, e.g., Ethernet on one end and Frame Relay on the other. In IPv4, each Provider Edge (PE) device discovers the IP address of the locally attached Customer Edge (CE) device and distributes that IP address to the corresponding remote PE device. Then each PE device responds to local ARP requests using the IP address of the remote CE device and the hardware address of the local PE device. In IPv6, each PE device discovers the IP address of both local and remote CE devices and then intercepts local Neighbor Discovery (ND) and Inverse Neighbor Discovery (IND) packets and forwards them to the remote PE device.[16]


Inverse ARP and Reverse ARP

Inverse Address Resolution Protocol (Inverse ARP or InARP) is used to obtain Network Layer addresses (for example, IP addresses) of other nodes from Data Link Layer (Layer 2) addresses. It is primarily used in Frame Relay (DLCI) and ATM networks, in which Layer 2 addresses of virtual circuits are sometimes obtained from Layer 2 signaling, and the corresponding Layer 3 addresses must be available before those virtual circuits can be used.[17]

Since ARP translates Layer 3 addresses to Layer 2 addresses, InARP may be described as its inverse. In addition, InARP is implemented as a protocol extension to ARP: it uses the same packet format as ARP, but different operation codes.

The Reverse Address Resolution Protocol (Reverse ARP or RARP), like InARP, translates Layer 2 addresses to Layer 3 addresses. However, in ARP the requesting station queries the Layer 3 address of another node, whereas RARP is used to obtain the Layer 3 address of the requesting station itself for address configuration purposes. RARP is obsolete; it was replaced by BOOTP, which was later superseded by the Dynamic Host Configuration Protocol (DHCP).[18]

ARP spoofing and Proxy ARP

A successful ARP spoofing attack allows an attacker to perform a man-in-the-middle attack.

Because ARP does not provide methods for authenticating ARP replies on a network, ARP replies can come from systems other than the one with the required Layer 2 address. An ARP proxy is a system which answers the ARP request on behalf of another system for which it will forward traffic, normally as a part of the network's design, such as for a dialup internet service. By contrast, in ARP spoofing the answering system, or spoofer, replies to a request for another system's address with the aim of intercepting data bound for that system. A malicious user may use ARP spoofing to perform a man-in-the-middle or denial-of-service attack on other users on the network. Various software exists to both detect and perform ARP spoofing attacks, though ARP itself does not provide any methods of protection from such attacks.[19]

Alternatives to ARP

Each computer maintains a database of the mapping of Layer 3 addresses (e.g., IP addresses) to Layer 2 addresses (e.g., Ethernet MAC addresses), which is maintained primarily by the reception of ARP packets form the local network link. Thus, it is often called the ARP cache. Traditionally, other methods were also used to maintain this table, such as static configuration files,[20] or centrally maintained lists.

Since at least the 1980s,[21] networked computers have a utility called 'arp' for interrogating or manipulating this table.[22][23][24]

ARP stuffing

Embedded systems such as networked cameras[25] and networked power distribution devices,[26] which lack a user interface, can use so-called ARP stuffing to make an initial network connection, although this is a misnomer, as ARP is not involved.

This is a solution to an issue in network management of consumer devices, specifically the allocation of IP addresses of ethernet devices where:

  1. the user doesn't have the ability to control DHCP or similar address allocation protocols
  2. the device doesn't have a user interface to configure it with
  3. the user's computer can't communicate with it because it has no suitable IP address.

The solution adopted is as follows:

  • The user's computer has an IP address stuffed manually into its address table (normally with the arp command with the MAC address taken from a label on the device)
  • The computer sends special packets to the device, typically a ping packet with a non-default size.
  • The device then adopts this IP address
  • The user then communicates with it by telnet or web protocols to complete the configuration.

Such devices typically have a method to disable this process once the device is operating normally, as it is vulnerable to attack.

Standards documents

  • RFC 826 - Ethernet Address Resolution Protocol, Internet Standard STD 37.
  • RFC 903 - Reverse Address Resolution Protocol, Internet Standard STD 38.
  • RFC 2390 - Inverse Address Resolution Protocol, draft standard
  • RFC 5227 - IPv4 Address Conflict Detection, proposed standard

See also


  1. American Power Corporation. "Switched Rack Power Distribution Unit Installation and Quick Start Manual" (PDF). Retrieved 2011-09-28.

This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.

External links