Technically it's not NAT64 today. Different prefix for one, but it's also not translated at the IP layer (yet). For TCP, we terminate the TCP in tailscaled and make a new TCP connection out and switch them together, so packets are not 1:1 end-to-end.
We also had grander plans for the 32 "site-id" bits in the middle there. Instead of just a 8-bit (now 16-bit) "site ID" number in there, you could actually put the 32-bit CGNAT IPv4 address of any peer of yours, and then access its IPv4 space relative to that node, without any configuration.
Say you have an Apple TV plugged in at home.
Then you're at a coffee shop and want to access something on your LAN and don't have a subnet router set up.
You should be able to `ssh 10-0-0-5-via-appletv.foo-bar.ts.net` and have MagicDNS map that "appletv" as the "Site ID" and put its 32-bit CGNAT address in, and then parse out the 10.0.0.5 as the lower 32-bits, and then have Tailscale route your packets via your home Apple TV node.
All subject to ACLs, of course, but we could make it a default or easy-to-enable recommended default that you could do such things as an admin for your self-owned devices.
So why it's called "4via6"? That was just kinda a temporary internal name that ended up leaking out to docs/KB and now a blog post, apparently. :)
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