ARP
Definition
The Address Resolution Protocol (ARP) is a communication protocol used for discovering the link layer address, such as a MAC address, associated with a given internet layer address, typically an IPv4 address. This mapping is a critical function in the Internet protocol suite.
Why It Matters
ARP is the bridge between the Network Layer (IP addresses) and the Data Link Layer (MAC addresses) on a local network. For a device to send a packet to another device on the same LAN, it must first use ARP to find the destination MAC address corresponding to the destination IP address.
Contextual Example
Your computer wants to send a packet to the router at IP 192.168.1.1. It broadcasts an ARP request on the local network saying, "Who has 192.168.1.1?". The router responds with an ARP reply saying, "I have 192.168.1.1. My MAC address is 00:1A:2B:3C:4D:5E." Your computer can now send the packet.
Common Misunderstandings
- ARP only works within a single local network; it does not work across routers.
- Devices maintain an "ARP cache" to store recently learned IP-to-MAC address mappings to avoid sending ARP requests for every packet.