> How do we teach every client on the Internet to talk to servers on public IPv6 addresses [and vice versa]?
> Answer: We go through every place that 4-byte IPv4 addresses appear, and allow 16-byte IPv6 addresses in the same place.
> ...
> Unfortunately, the straightforward transition plan described above does not work with the current IPv6 specifications. 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.
> ...
> This might sound like a very small mistake: after all, once IPv6 is working, we can move everything to IPv6, so who cares about IPv4? The problem is that this mistake has gigantic effects on the cost of making IPv6 work in the first place.
> How do we teach every client on the Internet to talk to servers on public IPv6 addresses [and vice versa]?
> Answer: We go through every place that 4-byte IPv4 addresses appear, and allow 16-byte IPv6 addresses in the same place.
> ...
> Unfortunately, the straightforward transition plan described above does not work with the current IPv6 specifications. 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.
> ...
> This might sound like a very small mistake: after all, once IPv6 is working, we can move everything to IPv6, so who cares about IPv4? The problem is that this mistake has gigantic effects on the cost of making IPv6 work in the first place.