The address resolution protocol (ARP) is used for

Answer: Finding the MAC address that corresponds to an IP address

Key idea: ARP (Address Resolution Protocol) maps IPv4 addresses to MAC (hardware) addresses on the local network so IP packets can be encapsulated into link-layer frames.

• When a host wants to send an IP packet to an IP on the same local network, it needs the destination's MAC address to build the Ethernet frame.

• If the sender does not have the MAC for that IP, it broadcasts an ARP request: "Who has <target IP>? Tell <sender IP>."

• The host owning that IP replies with an ARP reply containing its MAC address.

• The requester stores the IP-to-MAC mapping in its ARP cache for future use.

Why the other choices are incorrect:

• Finding the IP address from the DNS is incorrect because DNS translates domain names to IP addresses; it does not deal with MAC addresses or local link-layer resolution.

• Finding the IP address of the default gateway is incorrect because ARP does not discover IP addresses. However, if you know the gateway's IP, ARP can find its MAC so frames can be sent to it.

• Finding the IP address that corresponds to a MAC address is the inverse of ARP. Reverse mappings historically used RARP, but ARP itself resolves IP to MAC.

Additional notes: ARP operates within the local broadcast domain (it is not forwarded by routers). ARP caches improve efficiency but can be a target for spoofing attacks (ARP poisoning).