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We would have had it a long time ago if it had been backwards compatible, so that a server with an ipv6 address could talk with a server with an ipv4 address and vice versa. Then it would only have required software upgrades, but since ipv6 is more than twenty years old, most software that used it would have supported it by now, if only because it would have been written a long time after ipv6.

I've heard this argument before … but how could that have possibly worked? An IPv4 client is only going to be able to address 2 ^ 32 addresses, and it seems like the pigeonhole principle implies that the client can't possibly address the entire IPv6 space with that address format.

How would an IPv4 client, with this hypothetical backwards-compatible IPv6, connect to an IPv6 server?

A special flag in the ip packet that indicates which type of address it should be interpreted as, really that should be all, the rest of the work is drivers for software and routers.

If you need to change software on all machines and routers, what have you gained vs IPv6 as it is? being backwards-compatible would mean that it would work with unchanged old machines, which doesn't work since that wasn't planned in IPv4. IPv6 could be simpler, yes, and has accumulated quite a bit of well-meant baggage, but you don't get out of updating everything if you want to completely switch.

It is fairly easy to introduce a proxy for IPv4 only networks. Like a NAT gateway, but even easier since you can do this at the application level (and get real firewalling).

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