ARP - Address Resolution Protocol

ARP stands for Address Resolution Protocol but do you also wonder, what is ARP and how does ARP work? In this blog, we will be discussing these topics in detail. 

Have you ever imagined how the information or data is being shared over the internet? 

Any information exchanged from one source to destination with the help of IP, travels over the internet. We all know that an IP address helps to make the communication from source to destination. Have you ever wondered that IP Address is somewhat which works on routers but in today's production networks, we have a network device (Switch) that connects all the multiple end devices to make the communication. 

Now in the case of LANs, the devices are connected with a centralized network device known as a Switch, and the Switch is a device that doesn't understand IP Addresses as it's a layer-2 device of OSI, which works on frames. So, it works with the MAC address. 

In Local Area Networks, the communication is possible because of MAC Addresses, as these layer-2 switches can identify only the MAC Address. Now how they identify the MAC-Addresses of clients and how the communication is done, let's see that.

What is ARP?

As discussed earlier, the full form of ARP is Address Resolution Protocol. It is a communication protocol used to find the MAC-Addresses of the devices with the help of an IP Address. 

The primary purpose of the Address Resolution Protocol can be defined as to convert 32-bit addresses to 48-bit addresses and vice versa. The IP Addresses in IPv4 are 32-bits, while the MAC addresses are 48-bits. 

How does ARP work?

To understand how does ARP work, let's take a scenario:

In this scenario, we have switch and this switch is connected with 3 PCs with following IP's as – 

• PC-1 (192.168.1.1) 
• PC-2 (192.168.1.2) 
• PC-3 (192.168.1.3) 

Now, PC-1 (192.168.1.1) wants to communicate with PC-2(192.168.1.2). So, let's verify the MAC Table in switch first by using the command. 

Switch#show mac address-table

As you can see, no entries are there in MAC table of switch. Now, once the PC-1(192.168.1.1) drops a ping request to PC-2(192.168.1.2), the switch is going to store the MAC Address of PC-1(192.168.1.1) in its MAC table with the connected port number.

• As you can see now, with a PING request, one ARP request has been created. 
• These are the layer-2 & 3 headers information when an ARP request is created.

Now, when the PC-1(192.168.1.1) sends a ping request, the switch will store the MAC Address of PC-1 in its MAC table. See the image to verify.

And then, the switch, after storing the MAC of PC-1, will broadcast the frame in the network, and all the devices will reject, excluding the one whose destination is in a layer-3 header which is PC-2(192.168.1.2). 

When PC-2 replies to the switch, then the switch will store the MAC of PC-2.

This is how ARP works. So, let's discuss the requirement of the Address Resolution Protocol. 

What is the Requirement of ARP in the Network?

End devices in the LAN communicate with the MAC addresses, not IP addresses. Once the MAC entries are there in the MAC table, the switch will not broadcast the traffic in the network. The communication will be in the form of unicasting. 

The purpose of ARP is to find the MAC Addresses of connected clients in the network and create MAC entries so that the switch doesn't need to broadcast again and again to find the destination. 

Types of ARP

There are four types of ARP (Address Resolution Protocol), which are: 

1. Proxy ARP 

2. Gratuitous ARP 

3. Reverse ARP 

4. Inverse ARP 

Proxy ARP: It is used by the proxy servers on a network to answer the ARP queries for an IP address that is not available on that network. The proxy offers its own MAC address as the destination, and it routes the traffic to the intended destination via a tunnel or another interface. 

Gratuitous ARP: It is a special kind of ARP reply which is not a response to an ARP request. It is used in advanced network scenarios and is useful to detect IP conflicts. 

Reverse ARP: It is used by a device in the local area network, which knows its MAC address but isn't aware of its IP address. Reverse ARP requests the device's IP address from the gateway router's ARP table. 

Inverse ARP: As the name suggests, inverse ARP is the opposite of ARP. It uses the MAC addresses to find the IP addresses. 

These are the various types of ARP. We hope you have found the information you were looking for. Please comment on any suggestions or updates in the comment box below; we appreciate your valuable feedback. If you like this blog, you can subscribe to our free newsletter to never miss out on any updates.

FAQs Related to Address Resolution Protocol

Question – What is an ARP used for? 

Answer – An ARP is used for mapping an ever-changing IP address to a permanent MAC address inside a LAN (local area network). It is a very important procedure because the length of the two addresses differs from each other. 

Question – What is ARP and how it works? 

Answer – ARP stands for Address Resolution Protocol; it is one of the most important layer 3 protocols as it helps to find the MAC address by providing the IP Address. It works by converting the 32-bits of IP Address into the 48-bits of MAC Address. 

Question – How and where is ARP used? 

Answer – ARP is used when a device on a local area network wants to communicate with another device on the same local area network. The device uses the ARP to convert the IP address into MAC addresses.