Where is the IHL value stored in the IP header, and how is it used to determine the header length?

• A. The high-order nibble of the first byte, multiplied by 4.
• B. The low-order nibble of the first byte, multiplied by 4.
• C. The low-order nibble of the last byte, multiplied by 4.
• D. The low-order nibble of the first byte, multiplied by 4 to yield the header length in bytes.

Answer: (D)

The IHL value is stored in the low-order 4 bits of the first byte of the IPv4 header. That 4-bit field tells how many 32-bit words make up the header. To get the header length in bytes, multiply that value by 4. For example, if IHL is 5, the header length is 20 bytes (5 words × 4 bytes each). The high-order nibble of the first byte is the IP version, not IHL, and the IHL is not in the last byte, so the correct description is the low-order nibble of the first byte multiplied by 4 to yield the header length in bytes.