Address Resolution Protocol (ARP)

ARP maps a known IP address (logical, network layer) to the corresponding MAC address (physical, data link layer) on the same LAN. A host needs the destination MAC to build an Ethernet frame.

Process

1. Host A wants to send to Host B (IP known) and checks its ARP cache.
2. If there is no entry, A broadcasts an ARP Request (destination MAC FF:FF:FF:FF:FF:FF) containing A's IP/MAC and B's IP, asking "Who has IP B?"
3. All hosts receive it; only B recognizes its own IP.
4. B sends a unicast ARP Reply with its MAC address to A (and may cache A's mapping).
5. A stores the mapping in its ARP cache (with a timeout) and sends the frame.

Notes

• Request: broadcast; Reply: unicast.
• If the destination is on another network, the host ARPs for the default gateway's MAC instead.
• ARP packet fields: hardware type, protocol type, hardware/protocol lengths, operation (1=request, 2=reply), sender MAC/IP, target MAC/IP.