Address Resolution Protocol (ARP)

Address Resolution Protocol (ARP) is a computer networking protocol used to resolve a media access control (MAC) address from an Internet Protocol (IP) address. It is largely used on local area networks (LANs) and is a critical component of the overall network layer in the Internet Protocol Suite. The command-line tool used to manipulate ARP is called arp.

ARP operates between the layers of the network model, allowing mapping of layer 2 network addresses (MAC) to layer 3 IP addresses. It is a connectionless protocol and has no handshaking dialogues. It is a broadcast protocol, which means that it sends its request out to a broadcast address, and all other hosts in the network receive it.

The sender of the ARP request will send out packets with the details of the IP destination address and the sender’s hardware address. The recipient of the request will then reply by sending back its hardware address. This is how the sender translates the IP address into a hardware address.

ARP is not usually required for normal Ethernet-connected hosts because the hardware address is stored in a special logic chip called a field-programmable gate array (FPGA) or a network information device (NID). In more complicated architectures, such as virtualization, this logic chip might not be present and an ARP table is necessary in order for traffic to reach its destination.

ARPs are usually implemented as part of an operating system, but may alternatively be done at the firmware level for network-attached storage systems.

The Internet Engineering Task Force (IETF) maintains a list of all the registered address resolution protocols. Additionally, the IEEE maintains the IEEE 803.2 (Ethernet ARP) standard.

ARP is sometimes referred to as a crude level of security, because it relies on the sender verifying the IP-MAC address mappings, but it is easily spoofed. Other security protocols, such as anti-spoofing filters, should be used along with ARP for network security.