[zkms]

> The IPv6 designers made a fundamental conceptual mistake: they designed the IPv6 address space as an alternative to the IPv4 address space, rather than an extension to the IPv4 address space.

I respect djb and his contributions to cryptography, but he is off base here. The sin was committed when IPv4 was made and not initially designed to allow for variable / expanded address space.

Adding an IP Option to IPv4 packets that could carry extra address bits was not an option either -- IP options aren't preserved much at all on the Internet. Furthermore, even if most routers didn't drop IP options, adding "v6" address space via IP option in a packet that old/v4-only devices would nevertheless attempt to parse would have been hell operationally.

IPv6 has lots of flaws/idiosyncrasies/weirdnesses (multicast, mobility, slaac, ndp, etc.) that only looked good in the 90s, that definitely made ipv6 adoption a lot more painful than it needed to be, and ended up as difference for just difference's sake; but the one thing that was unambiguously done right with v6 was a completely clear separation of address spaces from IPv4.