ARP is a protocol used to match an IP address with a hardware address on a local network. When a device needs to send an IP packet, it broadcasts a message to the LAN requesting the hardware address of the destination IP address. The response is sent directly to the requester, and the addresses are stored in the ARP cache for later use. ARP supports IPv4 and Ethernet addresses, while IPv6 addresses are resolved with NDP. RARP and IARP are obsolete.
Address resolution protocol is a means for a network application or device to determine the Ethernet address of another device. Also known by the acronym ARP, it attempts to match an Internet Protocol (IP) address with a hardware address. ARP is used by each device on a local network the first time it communicates with another device on the same network.
Network devices and applications communicate at a high level using packets on the network layer of the protocol stack. These packets are directed to IP addresses. When each packet is sent over the local area network (LAN) using the data link layer, the recipient’s Ethernet address must also be known. IP addresses can be dynamically assigned and change over time, while Ethernet addresses generally stay the same. An application or device needs a way to determine the hardware address currently associated with a given IP address.
The Address Resolution Protocol defines a standard method for detecting these addresses. ARP was documented by the Internet Engineering Task Force (IETF) in 1982 as Request For Comments (RFC) 826. When a system is ready to send an IP packet, it consults its cache of recently resolved addresses. If the destination IP address is not in the cache, broadcast a message to the entire LAN using ARP. This message requests the device with the destination IP address to respond with its Ethernet hardware address.
This Address Resolution Protocol request packet includes the IP and hardware addresses of the requestor, as well as the IP address being sought. Each device on the LAN listens for an ARP request packet containing its IP address. If one is heard, the corresponding device composes an ARP reply packet containing its Ethernet hardware address. This response packet is then sent directly to the requestor, which stores the addresses in its ARP cache for later reuse. The responder may also store addresses in its cache, as it is likely to communicate further with the requester.
IP version 4 (IPv4) and Ethernet addresses are the most common types of addresses resolved by the Address Resolution Protocol. However, the request packet supports other types via the Protocol Type and Address Length fields. IP version 6 (IPv6) addresses are resolved with Neighbor Discovery Protocol (NDP) instead of ARP.
Reverse Address Resolution Protocol (RARP) requires an IP address for the requester’s hardware address. A RARP server responds to one of these requests by assigning a new IP address and associating it with the hardware address of the requestor. Reverse ARP is considered obsolete: its functionality is built into the Dynamic Host Configuration Protocol (DHCP). Inverse Address Resolution Protocol requires the existing IP address associated with the specified hardware address. In this case the hardware address can be that of any node of the same LAN.
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