ARP

ARP (Address Resolution Protocol)

ARP (Address Resolution Protocol) is a fundamental networking protocol used to map a logical IP address (Layer 3) to a physical MAC address (Layer 2) within a local network.

Even though ARP is closely related to IP, its packets are encapsulated directly inside Ethernet frames, which is why ARP is often described as operating at Layer 2.5 - a layer between the Data Link layer and the Network layer.

When a device wants to communicate with another device on the same local network, it must know the destination’s MAC address.

How it works

• If this address is unknown, the device sends an ARP request as a broadcast frame asking, “Who has this IP address?”.
• The device owning that IP responds with an ARP reply containing its MAC address.

This information is then stored temporarily in an ARP cache to reduce future requests. ARP is essential for local communication but does not provide security or reliability features, making it vulnerable to attacks such as ARP spoofing.