Wiki Ip Protocol Numbers And Assignment

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

ARP is used for mapping a network address such as an IPv4 address, to a physical address, such as a MAC address. 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[edit]

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[edit]

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 offset01
0Hardware type (HTYPE)
2Protocol type (PTYPE)
4Hardware address length (HLEN)Protocol address length (PLEN)
6Operation (OPER)
8Sender hardware address (SHA) (first 2 bytes)
10(next 2 bytes)
12(last 2 bytes)
14Sender protocol address (SPA) (first 2 bytes)
16(last 2 bytes)
18Target hardware address (THA) (first 2 bytes)
20(next 2 bytes)
22(last 2 bytes)
24Target 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[edit]

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[edit]

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[edit]

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[edit]

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[edit]

Main article: ARP spoofing

Main article: Proxy ARP

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[edit]

Each computer maintains a database of the mapping of Layer 3 addresses (e.g., IP addresses) to Layer 2 addresses (e.g., EthernetMAC 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[edit]

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[edit]

  • 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[edit]


  1. ^David C. Plummer (November 1982). "RFC 826, An Ethernet Address Resolution Protocol -- or -- Converting Network Protocol Addresses to 48.bit Ethernet Address for Transmission on Ethernet Hardware". Internet Engineering Task Force, Network Working Group. 
  2. ^Braden, R. (October 1989). "RFC 1122 - Requirements for Internet Hosts -- Communication Layers". Internet Engineering Task Force. 
  3. ^IANA ARP - "Protocol Type"
  4. ^IANA - Ethertype values
  5. ^RFC 5342
  6. ^"IANA ARP parameter assignments". IANA. 2009-04-24. 
  7. ^Chappell, Laura A. and Tittel, Ed. Guide to TCP/IP, Third Edition. Thomson Course Technology, 2007, pp. 115-116.
  8. ^Cheshire, S. (July 2008). "RFC 5227 - IPv4 Address Conflict Detection". Internet Engineering Task Force. 
  9. ^Perkins, C. (November 2010). "RFC 5944 - IP Mobility Support for IPv4, Revised". Internet Engineering Task Force.  
  10. ^Perkins, C. (October 1996). "RFC 2002 - IP Mobility Support". Internet Engineering Task Force. 
  11. ^Cheshire, S. (July 2008). "RFC 5227 - IPv4 Address Conflict Detection". Internet Engineering Task Force.  
  12. ^"FAQ: The Firewall Does not Update the Address Resolution Protocol Table". Citrix. 2015-01-16.  
  13. ^Gratuitous ARP in DHCP vs. IPv4 ACD DraftArchived October 12, 2007, at the Wayback Machine.
  14. ^RFC 2002 Section 4.6
  15. ^RFC 2131 DHCP – Last lines of Section 4.4.1
  16. ^Shah, H.; et al. (June 2012). "RFC 6575 Address Resolution Protocol (ARP) Mediation for IP Interworking of Layer 2 VPNs". Internet Engineering Task Force. 
  17. ^T. Bradley; et al. (September 1998). "RFC 2390 - Inverse Address Resolution Protocol". Internet Engineering Task Force. 
  18. ^Finlayson, Mann, Mogul, Theimer (June 1984). "RFC 903 - A Reverse Address Resolution Protocol". Internet Engineering Task Force. 
  19. ^Steve Gibson (2005-12-11). "ARP Cache Poisoning". GRC. 
  20. ^Sun Microsystems. "SunOS manual page for ethers(5) file". Retrieved 2011-09-28. 
  21. ^University of California, Berkeley. "BSD manual page for arp(8C) command". Retrieved 2011-09-28. 
  22. ^Canonical. "Ubuntu manual page for arp(8) command". Archived from the original on 2012-03-16. Retrieved 2011-09-28. 
  23. ^Apple Computer. "Mac OS X manual page for arp(8) command". Retrieved 2011-09-28. 
  24. ^Microsoft. "Windows help for arp command". Retrieved 2011-09-28. 
  25. ^Axis Communication. "Axis P13 Network Camera Series Installation Guide"(PDF). Retrieved 2011-09-28. 
  26. ^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[edit]

Internet Protocol version 4 (IP)

The Internet Protocol provides the network layer (layer 3) transport functionality in the InternetProtocolFamily.

This page describes IP version 4, which is widely used. There's also an IPv6 protocol page available.

The IP protocol is used to transfer packets from one IP-address to another. The user of this layer will give a packet and a remote IP address, and IP is responsible to transfer the packet to that host.

IP will (hopefully) guide the packet the right way to the remote host. The data transfer is independant of the underlying network hardware (e.g. ATM, Ethernet, or even a SerialLine). If the underlying hardware is not able to transfer the maximum length required (especially on SerialLine's or ATM), IP will split the data into several smaller IP fragments and reassemble it into a complete one at the receiving host.

When IP wants to send a packet on a LAN, it must first translate the IP-address given into the underlying hardware address (e.g. an Ethernet address). IP uses ARP for this translation, which is done dynamically. On a point-to-point line, this is obviously not necessary, as there's only one host to which a given machine can send a packet.

IP doesn't provide any mechanism to detect PacketLoss, DuplicatePackets and alike.

IP uses ICMP to transfer control messages to a remote host such as "Please don't send me more IP packets, I'm full". The famous ping tool also use ICMP.

The typical protocols on top of IP are TCP and UDP.

Version 4 of the IP protocol is widely used all over the world. As the available IP-address range is becoming short, version 6 with a much wider address range is becoming more and more popular these days.


The RFC791 "INTERNET PROTOCOL" was released in September 1981.

Protocol dependencies

  • Ethernet: IP can use Ethernet and many other protocols. The assigned Ethernet type for IP is 0x800.

  • ICMP: IP uses ICMP for control messages between hosts.

Example traffic

XXX - Add example traffic here (as plain text or Wireshark screenshot).


IP dissector is fully functional. Wireshark provides some advanced features such as IP defragmentation.

Preference Settings

  • Decode IPv4 TOS field as DiffServ field: Whether the IPv4 type-of-service field should be decoded as a Differentiated Services field (see RFC2474/RFC2475)

  • Reassemble fragmented IP datagrams: Whether fragmented IP datagrams should be reassembled
  • Show IP summary in protocol tree: Whether the IP summary line should be shown in the protocol tree
  • Validate the IP checksum if possible: Whether to validate the IP checksum
  • Support packet-capture from IP TSO-enabled hardware: Whether to correct for TSO-enabled hardware captures, such as spoofing the IP packet length
  • Enable GeoIP lookups: Whether to look up IP addresses in each GeoIP database we have loaded
  • Interpret Reserved flag as Security flag (RFC 3514): Whether to interpret the originally reserved flag as security flag

Example capture file

XXX - Add a simple example capture file. Keep it short, it's also a good idea to gzip it to make it even smaller, as Wireshark can open gzipped files automatically.

Display Filter

A complete list of IP display filter fields can be found in the display filter reference

  • Show only IPv4-based traffic (beware: you won't see any ARP packets if you use this filter!):


    Show only the IP-based traffic to or from host


    Show only the IP-based traffic to or from the subnet (The /24 is CIDR notation for a network address with a mask of 24 one bits, that is, a subnet mask of


    Show only the IP-based traffic not to or from host (beware: this is not identical to ip.addr!=


Capture Filter

  • Capture IPv4-based traffic only:


    Capture only the IP-based traffic to or from host


    Capture only the IP-based traffic to or from the subnet (The /24 is CIDR notation for a network address with a mask of 24 one bits, that is, a subnet mask of

    ip net

    Capture only the IP-based traffic not to or from host

    not host

External links


  • RFC894Transmission of IP Datagrams over Ethernet Networks

  • RFC950Internet Standard Subnetting Procedure

  • RFC1112Host Extensions for IP Multicasting

  • RFC1812Requirements for IP Version 4 Routers=== Differentiated Services (replaces Type of Service) ===

  • RFC2474Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers

  • RFC2475An Architecture for Differentiated Services


Internet_Protocol (last edited 2016-08-17 14:28:53 by LukasToggenburger)

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