[jedberg]

I assume the high bits would be in the variable length ip option field. So to an old stack it would look like a packet from 13.14.15.16 to 5.6.7.8. Both of those devices would have to be capable of routing ipv6 packets, but nothing in between would.

[vel0city]

Hmm, might work for the middle boxes that just do routing.

This would then only really work with both ends supporting this new IPv6 though. If one side only spoke OG IPv4 and the other side spoke this weird IPv6 with half its address bits in the header, there's a good chance the IPv4 side would just ignore the header bits on its return packets and thus wouldn't address it right. So, while that router in the middle might route these packets OK, you'd still practically need an IPv4 address to talk to other IPv4 devices. Which means we'd still have this mixed support, or that 5.6.7.8 would need to pretty much be a complete stateful NAT keeping track of connections for 6 to 4.

[jedberg]

> or that 5.6.7.8 would need to pretty much be a complete stateful NAT keeping track of connections for 6 to 4.

Yeah, that would be the transition period. Where any v6 router would also be a NAT gateway for any OG connections and would have to have both a v4 and v6 address.