Address Resolution Protocol

Address Resolution Protocol (ARP) is a protocol used for mapping a network address (such as an IP address) to a physical address, also known as a Media Access Control (MAC) address. This process is known as ARP resolution or ARP mapping.

ARP works at the data link layer of the OSI model and is used by network devices such as switches, routers, and computers to communicate with each other within a local area network (LAN). When a device needs to communicate with another device on the same LAN, it first checks its ARP cache to see if it already has the MAC address of the destination device. If the MAC address is not in the cache, the device sends an ARP request broadcast message to all devices on the network, asking for the MAC address associated with the IP address of the destination device. The device with the corresponding IP address then responds with its MAC address, allowing the original device to communicate with the destination device.

An example of how ARP works is as follows: let's say a computer with IP address 192.168.1.10 wants to communicate with another computer on the same network with IP address 192.168.1.20. The first computer checks its ARP cache to see if it has the MAC address of 192.168.1.20. If it doesn't, it sends an ARP request broadcast message to all devices on the network asking for the MAC address associated with 192.168.1.20. The computer with IP address 192.168.1.20 responds with its MAC address, and the first computer can then communicate with it using that MAC address.

ARP is vulnerable to several attacks, including ARP spoofing, in which an attacker sends false ARP messages to associate their own MAC address with the IP address of another device on the network. This allows the attacker to intercept and modify network traffic intended for the targeted device. ARP cache poisoning is another attack in which an attacker floods the network with false ARP messages, causing devices to update their ARP caches with incorrect information.

In ethical hacking, ARP can be used for reconnaissance, as the ARP cache of a target device can be queried to obtain a list of all devices on the same network. ARP spoofing can also be used to perform man-in-the-middle (MitM) attacks and intercept network traffic. To protect against these attacks, network administrators can use techniques such as ARP monitoring, ARP filtering, and static ARP table entries.