What is the fixed IPv6 header length?

• A. 20 bytes
• B. 40 bytes
• C. 60 bytes
• D. 80 bytes

Answer: (B)

The fixed IPv6 header length is 40 bytes. This base header is composed of specific fields with fixed sizes: the first 4 bytes carry the version, traffic class, and flow label; the next 2 bytes hold the payload length; the following byte is the next header, and another byte is the hop limit. After that come the source and destination addresses, each 16 bytes long. When you add them up (4 + 2 + 1 + 1 + 16 + 16), you get 40 bytes.

Extension headers can be added after the fixed header, which increases the total header length beyond 40 bytes. The Payload Length field indicates how many bytes follow the fixed header, including any extension headers. This is why 40 bytes is described as the fixed header length for IPv6.