Which statement about the IPv4 header checksum is true?

• A. The IPv4 header includes a checksum for the payload only
• B. The IPv4 header does not include any checksum
• C. The IPv4 header checksum is always zero
• D. The IPv4 header includes a header checksum field.

Answer: (D)

The key idea here is how the IPv4 header protections work. IPv4 uses a 16-bit header checksum to detect corruption in the header only. This checksum is stored in a dedicated field inside the header. It’s computed by taking the one's complement sum of all 16-bit words in the header with the checksum field set to zero, then taking the one's complement of that sum to place the result in the header.

Routers along the path may modify header fields (for example, TTL is decremented, or fragmentation bits change), so the checksum must be recomputed for each hop to keep header integrity checks valid. The payload itself isn’t included in this checksum; errors in the payload are typically handled by higher-layer checksums (like TCP/UDP) or other mechanisms.

Thus, the statement that the IPv4 header includes a header checksum field is true. The other options don’t fit because the checksum isn’t for the payload, there is a checksum in the header, and it isn’t always zero.