Why is ARP described as functioning at layer 2.5?

• A. It operates at Layer 2 only and has no IP involvement.
• B. It touches IP and MAC address information at both layers.
• C. It translates IP addresses to MAC addresses using a different protocol.
• D. It uses UDP ports for discovery.

Answer: (B)

ARP sits between layers 2 and 3 in practice. It exists so a host can discover the MAC address that corresponds to a known IP address, enabling a frame to be delivered on the local network. The reason it’s described as layer 2.5 is that ARP messages are carried inside layer 2 frames (Ethernet), but the information ARP uses and provides are IP addresses from layer 3. In an ARP exchange, the sender includes both its own IP address and its MAC address, and the target IP address; the target’s MAC is provided in the reply. This shows ARP directly involves both IP (layer 3) and MAC (layer 2) information, bridging the two layers. ARP requests are broadcast to reach the device with the matching IP, and the reply is sent back to the requester with the MAC address. It doesn’t rely on UDP or another protocol for discovery, and it isn’t purely a layer-2 operation.