ITPEC FE Morning October 2022 Question 31

ARP — the Address Resolution Protocol finds the MAC address that belongs to a known IP address on the local network.

Before a host can send a frame on a LAN it needs the MAC address of the target, or of the default gateway. It broadcasts an ARP request, "who has this IP address?", the owner answers with its MAC address, and the pair is cached in the ARP table for later frames.

Answer (a)

Why not others:

• •(b) ICMP — carries IP error and control messages (destination unreachable, the echo request and reply that ping uses); it resolves no addresses
• •(c) IP — routes packets by IP address; it relies on ARP to learn MAC addresses but does not resolve them itself
• •(d) RARP — works in the reverse direction, finding an IP address from a known MAC address (once used by diskless hosts at boot, now replaced by DHCP)

Key rule: ARP maps IP → MAC, RARP maps MAC → IP, and DNS maps a host name → IP.

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