ARP (Address Resolution protocol)

The address resolution protocol (ARP) is used to know to the receivers Mac address when the sender knows its  IP address

The above topology shows an L3 router with Mac address MR have three machines A, B, C  with Mac address MA, MB, MC, and all the four devices including the router falls in a subnet A which have a network id 12.1.1.0/24

Let us understand practically ARP with an example

From the above topology suppose, machine A likes to send data to machine B that means machine A will be a sending machine and machine B will be a receiving machine  

At this instant, machine A will not be in a position to send data to machine B until and unless it knows the B Mac address

Therefore, an ARP protocol is needed which allows machine A to know the machine B Mac address so, that machine A can talk to the machine B

Similarly, machine A should know machine C and L3 router Mac addresses to exchange data with them

ARP Table

ARP table  contains the mapping of IP to the Mac address of direct neighbors  here, the direct neighbors means all the machines that are available in the same subnet

So, by using ARP protocol machine A maintains IP to Mac address mapping of all other machines in the subnet, and this mapping is stored in a table called ARP table

Similarly, all the hosting machines in the topology, and  L3 router  which are present in the same subnet, will have a table called the ARP table

Example of ARP table of machines B and A

As you can see from the above ARP tables that it contains the mapping from IP address to Mac of all direct neighbors and also out going interfaces

An example

The above topology show the 3 machines VM, VM1, VM2 with subnet A and subnet B

ARP protocol operates only between the machines which falls in the same subnet

So, ARP will operate between machines VM and VM1

Why?

Because they have one interface each in the same subnet A(192.168.4.0/24)

Similarly, ARP  will also operate between the machines VM1 and VM2  because they also have one interface each in the same subnet B(192.168.1.0/24)

ARP will not operate between the machines VM and VM2

Why?

Because they don’t have any interfaces which are operating in the same subnet