What problem does ARP solve and how does an ARP request work?

• A. It maps IP addresses to MAC addresses; a host broadcasts an ARP request and the owner replies with its MAC.
• B. It maps MAC addresses to IP addresses; ARP request unicast.
• C. It translates hostnames to IPs; a DNS server responds with the IP, no broadcast.
• D. It encrypts IP packets; ARP requests are encapsulated in VPN tunnels.

Answer: (A)

ArP solves the need to translate an IP address into a MAC address so a local network frame can be delivered to the correct device. When a host wants to send to another host on the same Ethernet network and it only knows the destination IP, it sends out a broadcast ARP request asking “Who has this IP? please tell me your MAC.” The device that owns that IP responds with its MAC address in a unicast ARP reply to the requester. The requester then caches this IP-to-MAC mapping in its ARP table so future packets can be sent directly without another broadcast. This mechanism works only within the same broadcast domain; if the destination is on a different network, a router is involved. Note that ARP is used for IPv4; IPv6 uses a different mechanism called Neighbor Discovery.