The Address Resolution Protocol (ARP) is a fundamental protocol used in computer networks to map an IP (Internet Protocol) address to a physical (MAC) address. It operates at the Data Link Layer (Layer 2) of the OSI (Open Systems Interconnection) model and plays a crucial role in facilitating communication between devices within a local network.
The purpose of ARP is to resolve the layer 3 IP addresses to layer 2 MAC addresses. IP addresses are logical addresses assigned to network devices, while MAC addresses are unique physical addresses embedded in the network interface card (NIC) of each device. ARP provides a means to discover the MAC address of a device based on its IP address, enabling proper delivery of data packets on the local network.
ARP is a critical protocol for local network communication, allowing devices to dynamically discover the MAC addresses of other devices using their IP addresses. It facilitates efficient and accurate delivery of data packets within a network, making it an essential component of modern computer networking.
How ARP works
ARP Request: A device wants to send a data packet to a destination IP address but needs to know the corresponding MAC address.
The sending device checks its ARP cache (a table that stores recent IP-to-MAC mappings) to see if it already has the required entry.
If the entry is not found in the ARP cache, the sending device creates an ARP request packet. This packet contains the sender’s MAC and IP address, the target IP address, and a special ARP operation code indicating it’s a request.
The ARP request packet is encapsulated in a Layer 2 frame with the broadcast MAC address (FF:FF:FF:FF:FF:FF), indicating that it should be sent to all devices on the local network.
ARP Reply: All devices on the local network receive the ARP request packet but only the device with the IP address matching the target IP responds.
Upon receiving the ARP request, the device checks if the target IP matches its own IP address. If there’s a match, it creates an ARP reply packet.
The ARP reply packet contains the sender’s MAC and IP address, the target IP address, and a different ARP operation code indicating it’s a reply.
The ARP reply packet is encapsulated in a Layer 2 frame with the MAC address of the requesting device as the destination address.
The requesting device receives the ARP reply, updates its ARP cache with the IP-to-MAC mapping, and can now use the MAC address to send the data packet.
ARP Cache: Each device maintains an ARP cache, also known as the ARP table or ARP cache table.
The ARP cache stores recently resolved IP-to-MAC mappings to speed up future ARP requests and avoid unnecessary ARP broadcasts.
Entries in the ARP cache have a limited lifetime and may expire after a certain period of inactivity, requiring a new ARP request to refresh the entry.
ARP is a stateless protocol, meaning it doesn’t involve any form of session establishment or tracking. It relies on the broadcast nature of Ethernet networks to transmit ARP request packets and expects the intended recipient to respond with an ARP reply packet. This simplicity and reliance on the local network make ARP vulnerable to attacks like ARP poisoning, where malicious devices provide false IP-to-MAC mappings to intercept or manipulate network traffic.