ARP is a layer 2 protocol, used for obtaining MAC address of any devices within a network. Host machines use ARP protocol to obtain MAC Address. ARP protocol in conjunction with Layer 3 IP Protocol addressing (IP Address).

Host machine uses ARP because when machine needs to send packet to another device, destination MAC address is needed to be written in packet sent, so host machine should know the MAC Address of destination machine. Operating Systems also maintain ARP Table (MAC Address Table).

To obtain MAC address, ARP performs following process: (ARP request by host machine)

• Source machine generate ARP REQUEST packet with source MAC address (of this machine), source IP address (of this machine) and destination IP address and forwards this packet to switch.
• Switch receives the incoming packet and reads the source MAC address and checks its MAC address table, if entry for packet at incoming port is found then it checks its MAC address with the source MAC address and updates it, if entry not found then switch add and entry for incoming port with MAC address.
• All ARP REQUEST packets are broadcasted in network, so switch broadcast ARP REQUEST packet in network.

(Broadcast are those packets which are sent to everyone in network except the sender, only in network to which it belongs, it cannot span multiple networks)

• All devices in network receives ARP packet and compare their own IP address with the destination IP address in that packet.
• Only the machine which matches the both will reply with ARP reply packet. This packet will have source IP of this machine (which was destination machine in previous packet, as now its replying this machine will be the source machine) , source MAC address, destination MAC address (same as source MAC address in REQUEST packet) and destination IP address (same as source IP address in REQUEST packet).
• Then switch reads the ARP reply message and add entry in its MAC Address Table for port number on which it has received packet by reading its source MAC address field and forwards that packet to destination machine (source machine in REQUEST packet) as its MAC is in destination MAC address.
• Further host machine add destination machine entry into its ARP table.

Using this ARP protocol devices in network obtain MAC address of any other device in a network. Remember ARP works on broadcast, so it works only in a single network (local network).

To Understand How Networks Works on Layer 2 (Local Network) click here….