> No, backwards compatibility is competence.

Creating an IPng was always going to be complicated:

      So it's a matter of mathematical and physical fact that to expand
      the address size, you must change the protocol, and that means 
      [three] things immediately:

            You have to change the version number.

            You have to add new code to handle the new version.

      […]

            You need a coexistence technique so that updated systems, 
            with the new protocol, can connect to old systems that know 
            nothing of the new protocol.

      Two minutes of thought show that this third requirement has only 
      two solutions:

            (3A) Dual stack, in which the new machines speak both 
                 the old (IPv4) and new (IPng) protocol.

            (3B) Translation, in which something translates addresses
                 between the old and new protocols.

* https://github.com/becarpenter/misc/blob/main/why6why.md

* https://datatracker.ietf.org/doc/html/rfc1671

For anyone curious as to the decision-making process, here are the criteria for what the IETF was looking for in an IPng:

* https://datatracker.ietf.org/doc/html/rfc1726

Here are the candidates that were floating around at the time, and the reason why the winner was chosen:

* https://datatracker.ietf.org/doc/html/rfc1752

It's impossible for a long address system to be compatible with a short address system. No one has ever found a way to make it work. Plenty of people think they have, but upon closer inspection, they haven't.

NAT is the only one that actually does work, and it's terrible.