Address Resolution Protocol | ARP Table

Address Resolution Protocol

Fig 1: Address Resolution Protocol

ARP protocol is the protocol that stands for the Address Resolution Protocol and this protocol is originally  a data link layer protocol means it is a layer 2 protocol and  it is also known a layer 2.5 protocol because of it's working between the network layer and data link layer. So the main purpose of this protocol is find out the Mac Addresses that are the hardware addresses with the help of the IP addresses. This protocol is the protocol that is used that time when one node/device wants to communicate with the other device in the Local Area Network. Here below is the small topology of  local area network for the understanding of the ARP protocol how it's work.

Fig 2: Basic LAN Topology

The scenario is when  PC0 device that IP is 192.168.1.1 is want to communicate with the Laptop1 that IP is 192.168.1.4  then PC0 request go towards the switch and the  switch don't know about the destination where is the 192.168.1.4 and then after that one request is generate that is known as the ARP request in which following are the five terms exist:

• Source IP
• Source MAC Address
• Destination IP
• Destination Mac Address
• Payload

In our Scenario source IP is 192.168.1.1 and source MAC address of the PC0 that can be added in ARP request and after that destination IP that is 192.168.1.4 and the destination Mac address empty that is unknown and payload is the field. First of all when destination Mac Address is unknown then ARP request send to all the devices that exist in the network means that is the broadcast and the device that has the IP address 192.168.1.4 means Laptop1 respond this request and the reply of ARP request is ARP reply and the ARP reply contain the same terms that are following:

• Source IP
• Source MAC Address
• Destination IP
• Destination Mac Address
• Payload

And now the source become destination and destination become source and at that time all the terms are complete and this is the time when ARP Table create this table contains on the IP addresses and Mac addresses FF:FF:FF:FF:FF:FF and now the reply is the unicast because of reason of the known Mac Address. Here below is the ARP Table and the command for show ARP table is "arp -a"  on PC.

Fig 3: ARP Table

So after the creation of the ARP table the MAC table create in which there are the port numbers and their attach machine's MAC addresses and also the info of the VLAN.