What is ARP Protocol? How Devices Find Each Other on Networks

Ever thought about how the devices connected on the same local network actually communicate? In case of software, where IP addresses are used for routing data packets on the Internet, hardware of the local networks require physical addresses for delivery of packets of data to the right destination. The bridging of these processes takes place automatically through the use of Address Resolution Protocol (ARP).

The Basics of ARP: What is the ARP Protocol?

To understand what is ARP protocol, it is necessary to examine the way local networks communicate with the help of two different identification schemes:

• IP Address (Logical Layer): Dynamic address assigned to any device on a network (as a street address).
• MAC Address (Physical Layer): Unique, permanent hardware address programmed into the network card of the device (as a device’s digital fingerprint).

In other words, the ARP can be considered as a kind of translator for local networks. It provides a connection between Layers 3 (the Network Layer) and Layer 2 (Data Link Layer) of the OSI model, converting varying IP addresses into permanent physical MAC addresses.

Process of Address Resolution Protocol (ARP) Address Translation

If a laptop wants to communicate with a printer on the same network, it should know the printer’s hardware address. Here is the process of finding the required address:

• Looking for the Entry: The transmitter first looks for the corresponding MAC address IP mapping in its ARP cache (a temporary database).
• Sending the Broadcast: In case it does not exist in the table, the transmitting device sends a broadcast ARP request saying “who owns this IP address?”.
• Getting a Response: The receiver identifies the IP address and replies to the sender with its MAC address.
• Storing the Address: The sender saves the found address in the ARP cache and transfers the packets directly.

Conclusion

Understanding the ARP protocol makes us aware of the reason behind the smooth functioning of local networks as well. With the help of correct IP mapping with MAC addresses, ARP protocol makes sure that all the data going through local networks reach their correct destinations.

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