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IPv6 had an implicit bridge called 6to4 but it was phased out because it wasn't that reliable.

There was no upside for an ISP to operate a 6to4 relay, so the anycast address was worse than a blackhole route. It would occasionally permit a connection, but mostly just caused timeouts. Without that poison pill, it could have been a nice way to allow for connecting private subnets that otherwise use conflicting RFC1918 subnets. That's what I used it for.

Two 6to4 networks will communicate directly between each other without using a relay, so it will still work for that. Although you ought to be able to use native v6 these days.

If you can't deploy v6 (whether native or 6to4) on the remote side for whatever reason, NAT64 is useful for dealing with conflicting RFC1918. You map each instance of RFC1918 you need to access into different v6 /96s, and then they don't conflict from your perspective. (But like NAT44, it only works for outbound connections; inbound ones need a port forward.)

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