What is ARP (Address Resolution Protocol) ?

What is ARP (Address Resolution Protocol) ?

ARP (Address Resolution Protocol) is an internet layer protocol of TCP/IP protocol stack. The use of ARP is find out destination layer 2 address when layer 3 address is known. In other words, It helps to find out other devices on the same broadcast domain. For communication, on the ethernet, within broadcast domain, device must have some following address.

•     Source IP Address or any routed protocol address
•     Destination IP Address or any routed protocol address
•     Source MAC address 
•     Destination MAC address

If device knows the destination IP address but MAC address is unknown, ARP helps to find out the MAC address  of destination device. Basically, the IP address you want to reach you must know MAC address of the device. When a device want to communicate to another device, It checks the mac-address of target IP Address in its own local connection table. Look at the picture below for an example. PC-A wants to communicate with PC-B. PC-A doesn’t have any entry for PC-B in its local connection table.

 

Now, in this situation, PC-A needs to know MAC-addres of PC-B. Beacuse it has no entry for 192.168.1.2 (PC-B) in its local connection table. Now It will send a ARP request. Arp request is broadcast request. It uses destination IP address (in this case192.168.1.2)  and broadcast destination mac-address FFFF.FFFF.FFFF

 

Every Connected device on the broadcast domain will receive the request. In this PC-B, PC-C and PC-D will receive the frame. Except PC-B all other devices will drop the request. Because at the layer 3 destination IP will not match. PC-B receives ARP request because layer 3 address is match to 192.168.1.2 and PC-B Updates its own local connection table with Ip address 192.168.1.1 and mac-address abcd.0001.abcd

Since PC-B receive ARP request packet, It sends an ARP replay packet to PC-A using unicast addressing. Because PC-B has addresses of PC-A in their local connection table.

 

PC-A now receives ARP reply packet and updates local connection table with ip address 192.168.1.2 and abcd.0001.bbbb
Now PC-A has MAC-Address of PC-B and PC-A can Communicate with PC-B.

Summery of ARP

• ARP Stands for Address Resolution Protocol
• It works on layer 3
• It finds destination Layer 2 address if layer 3 address is known in the broadcast domain.
• It sends ARP Request on broadcast layer 2 address with target Ip address
• Only Target Ip address receives and sends ARP reply
• ARP Reply is sent out unicast.
• Defined in RFC 826