Where is the 16-bit header checksum located and when is it recalculated?

• A. The 16-bit Header Checksum field near the start of the header; recalculated whenever the header changes (e.g., TTL)
• B. The 32-bit Checksum field at the end; recalculated on every hop
• C. The 16-bit Fragment Offset field near the end; recalculated on receipt
• D. The 8-bit Type of Service field; recalculated on routing changes

Answer: (A)

The concept being tested is how IPv4 ensures the header’s integrity. The header checksum is a 16-bit field located in the header (near the start, after the Protocol field). It’s computed as a ones’ complement sum of all 16-bit words in the header, with the checksum field itself set to zero during calculation. Because any change to the header—such as a router decrementing the Time to Live (TTL) or other routing/fragmentation adjustments—changes the data that goes into that sum, the checksum must be recalculated to reflect the new header contents. That’s why the checksum is updated whenever the header changes, not on every hop for other fields, and not for payload changes. The other options point to fields that aren’t the header checksum, or suggest recalculation at incorrect times.