Which action does a router take regarding the IP header when forwarding a packet?

• A. Recomputes the UDP checksum only
• B. Validates the IP checksum, decrements TTL, and recalculates the IP header checksum
• C. Ignores the IP header checksum
• D. Only decrements TTL and forwards

Answer: (C)

When a router forwards an IPv4 packet, it must adjust the header to reflect the hop. The Time To Live (TTL) field is decremented by one, and because that changes the header, the IP header checksum must be updated to keep the header intact and valid. Routers typically also check the IP header checksum to detect corrupted headers and will drop packets with an invalid checksum. The UDP checksum isn’t recalculated just because the packet was forwarded, since the TTL change doesn’t affect the UDP checksum.

So the correct behavior is to validate the IP header, decrement TTL, and recalculate the IP header checksum. It’s not correct to say the router ignores the IP header checksum, and UDP checksums aren’t touched by this forwarding action.