How ARP Works: Address Resolution Protocol
ARP is a protocol used to map a known IP address to an unknown MAC address on a local area network, allowing for frame delivery.
The Purpose of ARP
Devices communicate locally using MAC addresses (Layer 2), but applications and routing use IP addresses (Layer 3). When a PC wants to talk to a local server, it knows the server's IP but needs its MAC address to build the ethernet frame. **Address Resolution Protocol (ARP)** solves this.
The ARP Process
1. The sender broadcasts an **ARP Request**: 'Who has IP 192.168.1.5? Tell 192.168.1.10.' 2. The request reaches all devices on the local subnet. 3. The device with the matching IP sends a unicast **ARP Reply** containing its MAC address.
The ARP Cache
To avoid broadcasting for every packet, devices store resolved IP-to-MAC mappings in an **ARP Cache** (or ARP Table) for a set period. You can view this cache on a Windows PC using the `arp -a` command, or on a Cisco router using `show ip arp`.
❓ Frequently Asked Questions
Is an ARP Request broadcast or unicast?
An ARP Request is sent as a Layer 2 broadcast (FF:FF:FF:FF:FF:FF).
Can ARP cross a router?
No, ARP requests are broadcast traffic, and routers do not forward broadcasts.
What happens if the destination is on another network?
The host will ARP for its Default Gateway's MAC address instead of the destination.