What is ARP?
Address Resolution Protocol (or ARP as it is commonly known) is used on Ethernet networks to assign IP addresses to globally unique MAC addresses. In this way, when a new system enters the network, its MAC address is assigned an IP address within the range of IP addresses on the network.
How Does it Work?
ARP uses a simple message format sent over the link layer and network layer (Layers 2 and 3 of the OSI model). This message contains one request or one response. For example, assume two computers on an Ethernet LAN. Computer 1 needs to send a packet to computer 2. Computer 1 knows that Computer 2’s IP address is 192.168.1.101. To send the packet to computer 2, it needs the physical address of computer two or its MAC address.
Computer 1 can find the MAC address of computer two by doing a lookup in the ARP table. The ARP table is a mapping of known physical addresses to logical addresses (MAC to IP). If computer 1 finds the MAC address of Computer 2 in the ARP table, it goes ahead and sends the packet to the MAC address of computer 2. If it doesn’t find the corresponding MAC in the ARP table, it sends out a broadcast ARP request to every computer on the network asking, “Who has IP address 192.168.1.101?”. Computer 2 will then send a unicast (1 to 1) response saying, “I have 192.168.1.101, and my MAC address is 11:22:33:44:55:66!”. Now Computer 1 can send the packet to MAC address 11:22:33:44:55:66.