What is a key difference between IPv4 and IPv6 headers?

• A. IPv4 headers are variable length (20-60 bytes) with 32-bit addresses; IPv6 headers are fixed at 40 bytes with 128-bit addresses and simplified fields.
• B. IPv4 headers fixed at 40 bytes; IPv6 headers vary.
• C. IPv4 uses 128-bit addresses; IPv6 uses 32-bit.
• D. IPv4 headers include a checksum; IPv6 headers also include a checksum.

Answer: (A)

The difference lies in how the header is designed and sized. IPv4 uses a variable-length header, 20 to 60 bytes, because it can include optional fields; it also uses 32-bit addresses. IPv6 changes this by adopting a fixed base header of 40 bytes and 128-bit addresses, with a streamlined main header and the use of extension headers for optional information. This makes processing more consistent and provides a vastly larger address space. The other options don’t fit because IPv4 headers are not fixed at 40 bytes and use 32-bit addresses, IPv6 uses 128-bit addresses, and IPv6 does not include a header checksum in the base header.