Which IPv6 header field is calculated as the payload size and is the evolution of the Datagram Length field?

• A. Header Length
• B. Payload Length
• C. Flow Label
• D. Next Header

Answer: (B)

Payload Length is the 16-bit field in the IPv6 header that specifies the length, in octets, of the payload following the IPv6 header. IPv6 uses a fixed 40-byte header, so there isn’t a Header Length field like in IPv4. The field that tells you how big the rest of the packet is—the payload after the header—is the Payload Length. This replaces the role of IPv4’s Datagram Length, which measured the total IPv4 datagram size (header plus data). The other fields aren’t about size: Header Length doesn’t exist in IPv6, Flow Label is for QoS tagging, and Next Header indicates what type of header follows rather than how long the payload is.