[ams6110]

If I'm at home and my computer has an IPv6 address, and I want to visit a site that has only IPv4 address, where does the NAT happen? Or does it happen? Or is it sort of like ASCII in UTF-8; all IPv4 addresses are directly representable in the IPv6 format? But even if that, how does the IPv4 service send a response back to me?

[tialaramex]

At home you will be "dual stack". That means your operating system implements an entire IPv4 network stack and an entire IPv6 stack, it will see this site only has IPv4, and use IPv4. If your IPv4 access goes through a NAT, then it will use the NAT exactly as if you didn't have IPv6 at all.

(Yes, all IPv4 addresses can be represented as IPv6 addresses in a reserved zero prefix network, some network APIs just offer IPv6 and then treat IPv4 addresses this way for convenience, but we obviously don't route packets this way since those would be IPv6 packets, yet they're for an IPv4 destination which can't read them)

In some very large deployments they do v6-only. Everything internally is IPv6, when you connect to that IPv4 only site you'd actually connect to a company-provided gateway that speaks IPv6 on your side but IPv4 to the outside world.

If you're big enough this makes loads of sense - now you have this vast address space for everything, all your systems are simply configured for IPv6 only (not twice the configuration) and it pretty much just works. You buy some specialist appliances at the edge for that translation, but your users only have IPv6 stuff.

[AtlasBarfed]

Wouldn't it have been nice for IPv6 to provide a basic translation protocol or mapping for seamless migration and backwards compatibility? Where your IPv4 address is also an IPv6 address?

I'd welcome someone showing me how that was basically impossible, but I don't see why the entire IPv4 space isn't in a special prefix / address space of ipv6 since they allegedly have so many atoms of mappings.

[fulafel]

This was done, it was called 6to4 and it worked pretty well when native IPv6 was less available. The gateways had an anycast IPv6 address too so you didn't have to manually configure the address of your gateway to the real IPv6 internet.