A host (192.168.1.10/24, default gateway 192.168.1.1) sends a packet to server 203.0.113.80 on a different network. With an empty ARP cache, whose MAC address does the host look up with ARP?

Answer: A. The default gateway, 192.168.1.1

The host first uses its own IP address and subnet mask to decide whether the destination is on the same network. 203.0.113.80 is outside 192.168.1.0/24, so the packet is handed to the default gateway (a router). The destination in the IP header stays the server's 203.0.113.80; only the frame's destination MAC address is set to the gateway's. So ARP looks up the MAC address of 192.168.1.1. ARP broadcasts do not cross routers, so the host cannot look up, and does not need, the MAC address of a server on another network. Each router along the path looks up the MAC address of the next device whenever it forwards.

• A. Correct. The frame goes to the gateway, so its MAC address is looked up.
• B. ARP broadcasts do not cross routers, so they never reach a server on another network.
• C. The host only needs the first router; each router looks up the next hop itself.