The IPv4 header checksum is computed over which portion of the IPv4 packet?

• A. payload only
• B. header and payload
• C. entire datagram
• D. header only

Answer: (D)

The main point is that the IPv4 header checksum protects just the header, not the payload. It’s computed as the 16-bit one's complement of the one's complement sum of all 16-bit words in the header, with the checksum field itself set to zero during calculation. Because routers may modify header fields on every hop (for example, TTL decreases, fragmentation-related fields can change with each fragment), the checksum must be recalculated for the header after any change so the header can be verified for integrity at each hop.

The payload isn’t included in this calculation, which is why the ISP or intermediary devices can’t rely on the IP header checksum to detect payload corruption. Payload integrity is handled by higher-layer protocols (like TCP or UDP) through their own checksums. If the question asks which portion is covered by the IPv4 header checksum, the correct answer is the header only, since that’s exactly what the IP header checksum is designed to protect.