What is Address Resolution Protocol (ARP)?

The Address Resolution Protocol (ARP) connects network addresses to physical addresses. It maps Layer 3 IP addresses to Layer 2 MAC addresses. Devices on a local network use MAC addresses to communicate. ARP bridges the gap between software and hardware addressing.

The ARP Request and Reply Cycle

When a device needs to send data, it checks its ARP cache. If the MAC address is unknown, it broadcasts an ARP request. This request asks, “Who has this IP address?” The device with the matching IP sends a unicast ARP reply. This reply contains its MAC address. The sender updates its ARP cache and transmits the data.

ARP Cache Timeouts and Security

ARP entries have a limited lifespan. Cache timeouts ensure devices do not hold outdated information. This dynamic system has vulnerabilities. Attackers can exploit it using ARP spoofing. In this attack, malicious actors send forged ARP messages. This links the attacker’s MAC address to a legitimate IP. This can lead to data interception or denial of service. Learn more about ARP standards at ietf.org.