ARP is a communication protocol used for discovering link layer address associated with a given internet layer address. Eg: find the MAC (media access control) addresses associated with IPV4 addresses. It is a request-response protocol whose messages are encapsulated by a link layer protocol. It is used only within the boundaries of a single network and never routed over internet.
Working:
Two computers in an office (Computer 1 and Computer 2) are connected to each other in a local area network by Ethernet cables and network switches, with no intervening gateways or routers. Computer 1 has a packet to send to Computer 2. Through DNS, it determines that Computer 2 has the IP address 192.168.0.55.
To send the message, it also requires Computer 2’s MAC address. First, Computer 1 uses a cached ARP table to look up 192.168.0.55 for any existing records of Computer 2’s MAC address (00:eb:24:b2:05:ac). If the MAC address is found, it sends an Ethernet frame with destination address 00:eb:24:b2:05:ac, containing the IP packet onto the link. If the cache did not produce a result for 192.168.0.55, Computer 1 has to send a broadcast ARP request message (destination FF:FF:FF:FF:FF:FF MAC address), which is accepted by all computers on the local network, requesting an answer for 192.168.0.55.
Computer 2 responds with an ARP response message containing its MAC and IP addresses. As part of fielding the request, Computer 2 may insert an entry for Computer 1 into its ARP table for future use.
Computer 1 receives and caches the response information in its ARP table and can now send the packet.
An ARP table is simply the method for storing the information discovered through ARP. It’s used to record the discovered MAC and IP address pairs of devices connected to a network. Each device that’s connected to a network has its own ARP table, responsible for storing the address pairs that a specific device has communicated with.
In IPv6 networks, the functionality of ARP is provided by the Neighbor Discovery Protocol (NDP) and its extensions such as Secure Neighbor Discovery.
