ARP stands for Address Resolution Protocol.
It is used by the Internet Protocol (IP) to map IP network addresses (32 bit Logical Address) to the hardware addresses (48 bit Mac Address) used by a data link protocol.
The protocol works below the network layer as a part of the interface between the OSI network and OSI link layer.
How it works:
Step 1: When a source machine wants to communicate with another machine, source checks its Address Resolution Protocol (ARP) table to find whether it already has a resolved MAC Address of the destination machine. If it is there, it will use that MAC Address for communication. Open Command Prompt and type command “arp -a” – it will show ARP table.
Step 2: If ARP resolution is not there in local table, the source machine will generate an Address Resolution Protocol (ARP) request message, it puts its own data link layer address as the Sender Hardware Address and its own IPv4 Address as the Sender Protocol Address. It fills the destination IPv4 Address as the Target Protocol Address. The Target Hardware Address will be left blank, since the machine is trying to find that.
Step 3: The source broadcast the Address Resolution Protocol (ARP) request message to the local network.
Step 4: The message is received by each device on the LAN since it is a broadcast. Each machine compare the Target Protocol Address (IPv4 Address of the machine to which the source is trying to communicate) with its own Protocol Address (IPv4 Address). Those who do not match will drop the packet without any action.
Step 5: When the targeted machine checks the Target Protocol Address, it will find a match and will generate an Address Resolution Protocol (ARP) reply message. It takes the Sender Hardware Address and the Sender Protocol Address fields from the Address Resolution Protocol (ARP) request message and uses these values for the Targeted Hardware Address and Targeted Protocol Address of the reply message.
Step 6: The destination device will update its Address Resolution Protocol (ARP) cache, since it need to contact the sender machine soon.
Step 7: Destination device send the Address Resolution Protocol (ARP) reply message and it will NOT be a broadcast, but a unicast.
Step 8: The source machine will process the Address Resolution Protocol (ARP) reply from destination, it store the Sender Hardware Address as the layer 2 address of the destination.
Step 9: The source machine will update its Address Resolution Protocol (ARP) cache with the Sender Hardware Address and Sender Protocol Address it received from the Address Resolution Protocol (ARP) reply message.
