&gt; 6in4 gives IPv4 hosts IPv6 addresses which means we only have to upgrade all the edges, but we still have to upgrade all of them.<p>No-one&#x27;s turning off their IPv4 stacks though. Under your proposal if one or other edge was IPv4-only then they&#x27;d use ordinary IPv4, you&#x27;d only see any benefit if both edges were IPv4+, right? Which is the same as when using 6to4. (If you really need to avoid dual-stack even on the local network you can use 6to4 and NAT64).<p>(6to4 is largely a failure but the kind of approach you propose would have the same issues as far as I can see)