Which statement best describes IPv6 header characteristics compared to IPv4?

• A. IPv6 headers are variable length and 128-bit addresses.
• B. IPv6 uses 32-bit addresses.
• C. IPv6 header length is flexible up to 60 bytes.
• D. IPv6 headers are fixed at 40 bytes with 128-bit addresses and simplified fields.

Answer: (D)

IPv6 standardizes a fixed 40-byte base header and uses 128-bit addresses, with the header fields simplified to speed up processing. This means every IPv6 packet has a predictable header size, and the more complex or optional information is carried in separate extension headers rather than in the base header. In contrast, IPv4 allows a variable header length (20 to 60 bytes) because options can extend the header, and IPv4 uses 32-bit addresses, with additional fields like a header checksum. The statement that best captures these characteristics is the one describing a fixed 40-byte header, 128-bit addresses, and simplified fields.