> Because the ipv4 is where all the stuff the ipv6 phone wants.
There's still some ipv4 only services, but most of the big ones are dual stack. Looks like right now tiktok is v4only, which is probably significant, but Google, Facebook, Netflix are dual stack. Amazon/EC2 have lots of v4 only bits and pieces, but at least www and cdn are dual stack. Github is also v4 only and that's important, but how many people are pulling from their phone?
> So here's a question: if your ipv6 is behind CGNAT and calls an ipv6 on the other side of the CGNAT: is it still one-way, or un-NAT'ed?
Depends, it's easy to do things like 464xlat and NAT64 where you route those address spaces through the CGNAT and other stuff direct. Or through a stateful firewall (which could be the CGNAT or something else) if you really need a stateful firewall.
There's still some ipv4 only services, but most of the big ones are dual stack. Looks like right now tiktok is v4only, which is probably significant, but Google, Facebook, Netflix are dual stack. Amazon/EC2 have lots of v4 only bits and pieces, but at least www and cdn are dual stack. Github is also v4 only and that's important, but how many people are pulling from their phone?