Address Resolution Protocol (ARP)

The Address Resolution Protocol (ARP) is a fundamental protocol used in the Internet Protocol Suite (TCP-IP) for finding the MAC Address of a device associated with a specific IP Address. It operates within the Data Link Layer of the Internet protocol suite and is used primarily in local area networks (LANs).

ARP's primary function is to translate 32-bit IP addresses into 48-bit MAC addresses. This is necessary because while IP addresses are used to identify devices on a network at the logical (software) level, MAC addresses are used to identify devices at the physical (hardware) level.

When a device (like a computer or router) on a LAN wants to communicate with another device, it needs to know the recipient's MAC address. If the sending device only knows the IP address of the target device, it broadcasts an ARP request onto the network. This request asks, "Who has IP address X.X.X.X, and what is your MAC address?" The device with that IP address responds with its MAC address. The requesting device then stores this mapping in its ARP cache for future reference and proceeds with communication.

Each device on a network maintains a table known as the ARP cache, which stores IP-to-MAC address mappings for a certain period. This cache reduces the need to repeatedly broadcast ARP requests for the same IP addresses.

ARP is essential in local area networking, particularly within Ethernet networks. ARP only works in the same broadcast domain or network segment. It is not routable, meaning it does not work across different networks.

ARP does not include a mechanism for authenticating ARP responses. This can be exploited in ARP Poisoning (or ARP poisoning) attacks, where an attacker sends false ARP messages to the network. This can lead to traffic interception or network disruption.