What operation is performed on each 16-bit word during IP checksum calculation?

Answer: A — Take 1's complement of each 16-bit word before summing

The key idea is that the IP header checksum uses a 16-bit ones’ complement sum across all 16-bit words in the header. You add each 16-bit word to a running total, and if a carry comes out beyond 16 bits, you wrap it back into the low 16 bits (end-around carry). After all words are added this way, you take the 1’s complement (invert all bits) of the final sum to produce the checksum. This means you’re not flipping each word before summing, nor using XOR, multiplication, or CRC. The process relies on ones’ complement arithmetic to ensure the checksum detects errors effectively.