[cesarb]

A NAT router is not enough. Suppose the v4 side wants to initiate the communication; to which address would it send the initial packet? Remember, the "v4 side" has no concept of extended addresses at all, for it every address must be 32 bits and nothing more. And it also doesn't solve the "v4 router in the middle of the path" problem.

[lisper]

> A NAT router is not enough. Suppose the v4 side wants to initiate the communication.

The router would also need to act as a DNS server and translate v6 responses into dynamically assigned v4 addresses. Routers routinely do this sort of thing today for captive portals.

[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.