Re: "We have now run out of IPv4 addresses"
>If the leading 4 bytes are padding then there's no point in them being there.
There was no point to IPv6 until now! ie. when the IPv4 address space becomes exhausted.
Back before the popularisation of the Internet to the masses in circa 1995, the academic institutions ran the show. So it was known that for the next 5+ years there would be no need to actually route outside of the IPv4 subnet of IPv6 addresses... So in this period a forced transitioned to IPv6 with everyone only using the IPv4 subnet was possible. Once the Internet went mass market and commercial entities got involved in running the backbone such a transition became much more problematic if not impossible.
>I don't think you could've done that in 1995. Even 1985 probably would've been too late for that.
I use circa, the key is some things were possible when the academic institutions ran the show and effectively could agree between themselves and tell everyone else what was going to happen and when commercial entities got involved in the mid-1990's. In any case those involved in the specification of IPv6 weren't going to be rushed into delivering 'IPv4 enlarged address space' and so the window of opportunity closed.
All of this baggage, historic design compromises just means TCP/IP is the networking world's QWERY keyboard.