[Dagger2]

But mine was that you don't need to do this as an intellectual exercise, because we got basically all the things you're asking for.

We have address extensions in v4 packets, we have NAT to help with partial upgrades, and we have a mix of 32-bit and 128-bit addresses (which should be just as obviously clean as a mix of 32-bit and 64-bit addresses, or rather more so due to 64-bits being too small). You don't need to think about whether any of this would have been doable, because we already went and did it.