What is Address Resolution Protocol?
4 min read
The Address Resolution Protocol (ARP) is a communication protocol used to map IP addresses to MAC (Media Access Control) addresses within a local network. In other words, it helps in translating network layer addresses (IP addresses) to data link layer addresses (MAC addresses). ARP acts like a translator in the world of computer networks. This translation is necessary because while devices communicate with each other using IP addresses, data packets are physically delivered based on MAC addresses.
A device’s IP address may change over time, but its MAC address is permanent because it is unique to a device's network interface card. ARP does not have a specific port number since it is a layer 2 protocol and port numbers are associated with the layer 7 application layer.
Function: ARP helps discover the physical address (MAC address) of a device on a local network (LAN) when you only know its logical address (IP address).
Think of it this way: Imagine you have a phone book with everyone's phone numbers (IP addresses) but not their physical addresses (MAC addresses). ARP acts like a directory service that helps you find the physical address (MAC address) given a phone number (IP address).
How does ARP work?
ARP operates at the Data Link Layer (Layer 2) of the OSI model.
It broadcasts a request message on the network asking for the device with a specific IP address.
The device with the matching IP address responds with its MAC address.
ARP is crucial for communication within a LAN. Without ARP, devices wouldn't know where to send data packets specifically because they only have IP addresses to identify each other.
If we can send data through an IP address. Then why do we even need ARP?
LAN devices can communicate on the Network layer (Layer 3) to a certain extent. Data is ultimately sent using IP addresses. However, ARP plays a vital role behind the scenes to ensure the data reaches the intended device. Devices on a LAN primarily use IP addresses for communication. They do leverage the Network layer for functionalities like:
IP header information: Data packets within a LAN contain IP headers with the sender's and receiver's IP addresses. This information is crucial for routing decisions within the network.
Subnet masks: Subnet masks within IP addresses help identify devices belonging to the same local network segment.
However Direct communication solely based on IP addresses within a LAN isn't sufficient due to Missing physical addresses. IP addresses don't directly translate to the MAC addresses of devices.
This is where ARP comes into play. It bridges the gap between the logical IP addressing system (Layer 3) and the physical addressing system (MAC addresses) used at the Data Link Layer (Layer 2).
Therefore, a combined approach is used:
IP for routing: Devices use IP addresses within the Network layer to identify and route data packets towards the destination network segment.
ARP for final delivery: Once the data reaches the destination network segment (LAN), ARP takes over. It translates the IP address into the specific MAC address of the intended device, enabling the data packet to be delivered directly to the correct recipient.
Conclusion
ARP is essential for the functioning of Ethernet networks because it allows devices to communicate with each other using MAC addresses, which are necessary for delivering data packets at the data link layer. Without ARP, devices wouldn't be able to determine each other's MAC addresses dynamically, leading to communication failures within the network.