[zlynx]

v4 systems can only talk to other v4 systems.

If there's a system that has only v4, it can tunnel v6 packets out to a tunnel gateway that will unwrap and forward them.

There are other protocol-specific tricks, like v4 DNS records that resolve to a HTTP reverse proxy, which forwards based on hostname and path to the real v6 server, while the v6 DNS records point directly to the v6 server.

[lisper]

> v4 systems can only talk to other v4 systems.

That depends on what you mean. A v4 packet can only be delivered unaltered to a v4 stack. But one can build a proxy server that tunnels TCP and UDP from a v4 network to a v6 network, so data can be passed from a v4 system to a v6 system without any software changes at either end.