ITPEC FE Subject A April 2025 Question 23

The destination IP address — a router forwards each packet by looking up where it is going in its routing table.

A router works at the network layer. For each incoming packet it reads the destination IP address, finds the best (longest) matching entry in its routing table, and sends the packet out of that entry's interface toward the next hop. The destination IP address stays the same from the source to the final host, so every router along the way can use it.

Answer (a)

Why not others:

• •(b) Destination MAC address — it only identifies the next device on the local link; on arrival it is the router's own interface, and it is rewritten at every hop, so it cannot say where the packet is ultimately going
• •(c) Source IP address — tells where the packet came from, not where it is going; ordinary routing ignores it
• •(d) Source MAC address — identifies only the previous device on the link, which says nothing about the destination

Key rule: IP addresses are end-to-end and used by routers (layer 3); MAC addresses are hop-by-hop and used by switches (layer 2).

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