Address Resolution Protocol (ARP)

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Introduction

ARP is a request-response communication protocol used to associate(map or translate) the IP network address to the Hardware or Physical or Media Access Control(MAC) address.
In another words, ARP is a type of resolution protocol used to dynamically map layer-3 network IP addresses to data-link hardware addresses.

ARP uses an IP address to find a MAC address.
It is one of the most important protocol of Network layer of OSI reference model.
It is used when IPv4 is used over Ethernet/LAN.
ARP translates the 32-bit IPv4 address into 48-bit MAC address.
The LAN keeps a table or directory(ARP table) that maps IP addresses to MAC addresses of the different devices, including both endpoints and routers on that network. This table or directory is not maintained by users or even by IT administrators. Instead, the ARP protocol creates entries on the fly. If ARP is not supported, manual entries can be made to this directory.

This protocol is used when a device wants to send data to another target device on Ethernet/LAN, it must first determine the MAC/Physical address of that target given its IP address. These IP-to-MAC address mappings are derived from an ARP cache maintained on each device. To do this, the initiating device first sends an ARP request broadcast message on the local subnet. The host with the given IP address sends an ARP reply in response to the broadcast via router, allowing the initiating device to update its cache and proceed to deliver messages directly to the target.

ARP helps in communicating with all the machines on the network i.e. this protocol is useful when one machine, with some IP address on an ethernet network, wants to communicate to another machine on the same network with different IP address.