ITPEC FE Subject A October 2025 Question 24

Topic: Networks

PC5 has been newly added to the network. It wants to send data to PC4 using SSH protocol. It must resolve the MAC address of PC4 from its IP address before sending the data. Which of the following is the destination MAC address that PC5 will use in ARP request?

NetworkSwitchPC1PC2PC3PC4PC5192.168.1.278:E1:00:3C:A1:5D192.168.1.578:E1:00:3C:A1:6E192.168.1.778:E1:00:3C:A1:7A192.168.1.1078:E1:00:3C:A1:8B192.168.1.1178:E1:00:3C:A1:9F

Explanation

FF:FF:FF:FF:FF:FF — an ARP request is a broadcast, because the sender does not yet know the target's MAC address.

PC5 knows PC4's IP address, 192.168.1.10, but not its MAC address. It sends an ARP request asking "who has 192.168.1.10?" to the broadcast address FF:FF:FF:FF:FF:FF, so the switch floods it to every port and every host on the segment receives it. Only PC4 recognises its own IP and answers with a unicast ARP reply carrying 78:E1:00:3C:A1:8B.

Answer (d)

Why not others:

  • •(a) 00:00:00:00:00:00 — the all-zero address is not a broadcast; it appears in the target MAC field inside the ARP payload as a placeholder for the unknown address, not as the frame's destination.
  • •(b) 78:E1:00:3C:A1:8B — PC4's MAC is exactly what PC5 is trying to learn, so it cannot address the request to it.
  • •(c) 78:E1:00:3C:A1:9F — this is PC5's own MAC, the source of the request.

Key rule: ARP requests are broadcast to FF:FF:FF:FF:FF:FF; ARP replies are unicast back to the requester.

Networks · AI-written, may contain errors

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