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What is Address Resolution Protocol (ARP)?

Address Resolution Protocol (ARP): Here, we are going to learn about the address resolution protocol, packet structure, etc.
Submitted by Anushree Goswami, on January 09, 2021

ARP: Address Resolution Protocol

ARP is an abbreviation of "Address Resolution Protocol". It is a communication protocol, linked with a specified internet layer address, in general, an IPv4 address used for finding out the link-layer address, such as a MAC address. In the Internet protocol suite, this mapping is a significant function.

ARP has been put into operation with a lot of amalgamation 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). The most widespread usage is IPv4 over IEEE 802.3 and IEEE 802.11.

In 1982, ARP was described by RFC 826, which is Internet Standard STD 37. The role of the task of ARP in Internet Protocol Version 6 (IPv6) networks are made available by the Neighbor Discovery Protocol (NDP).

Packet Structure

In this state of affairs, the packet has 48-bit fields for the sender hardware address (SHA) and target hardware address (THA), and 32-bit fields for the equivalent sender and target protocol addresses (SPA and TPA). In this condition, the ARP packet size is 28 bytes.

• Hardware type (HTYPE)
  This field indicates the network link protocol type. Example: Ethernet is 1.
• Protocol type (PTYPE)
  This field indicates the internetwork protocol for which the ARP request is planned or projected. The approved and allowed PTYPE values to contribute to a numbering space with those for EtherType.
• Hardware length (HLEN)
  It is the length of a hardware address in octets. Example: Ethernet address length is 6.
• Protocol length (PLEN)
  It is the length of internetwork addresses in octets. The internetwork protocol is indicated in PTYPE. Example: IPv4 address length is 4.
• Operation
  It indicates the operation that the sender is carrying out: 1 for request, 2 for the reply.
• Sender hardware address (SHA)
  
  1. It is the media address of the sender.
  2. In an ARP request, this field is used to specify the address of the host sending the request.
  3. In an ARP reply, this field is used to specify the address of the host that the request was searching for.
• Sender protocol address (SPA)
  It is the Internetwork address of the sender.
• Target hardware address (THA)
  
  1. It is the media address of the intended receiver.
  2. In an ARP request, this field is disregarded.
  3. In an ARP reply, this field is used to specify the address of the host that started off the ARP request.
• Target protocol address (TPA)
  It is the Internetwork address of the intended receiver.

Reference: Address Resolution Protocol

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