Which of the following is not a valid difference between IPv4 and IPv6? 

IPv6 explicitly supports header extensions to provide additional functionality like security, mobility, and routing. IPv4, on the other hand, has fixed header lengths and does not natively support the extension mechanisms present in IPv6. Other differences between the two include address length, NAT usage, IPsec inclusion, and address notation. IPv6 was designed to overcome the limitations of IPv4, making header extension a crucial improvement. Why other options are wrong: A) Correctly states the difference in address length. B) NAT is commonly used in IPv4 due to limited address space, while IPv6 has a much larger address space and does not rely on NAT. C) IPv4 addresses are written in dotted decimal format, whereas IPv6 uses hexadecimal. E) IPv6 requires IPsec for security, whereas IPv4 does not mandate it.