The original impetus behind IPv6 was to maintain the end-to-end principle, to allow clients and servers to directly address each other, if not communicate without mediation. But that's not the internet we built.

We're not at the stage many of us predicted in the 1990s or early 2000s where we might turn off IPv4 one day. A large number of mobile clients have IPv6; a large number of desktops and laptops don't; the backbone has been humming along for almost twenty years; and, on the server end, what was once a single machine is now often an SSL terminating load balancer.

We are, unquestionably, in a dual-stack world, and it looks like we've settled into that mode for a long, long time. So what are the rules now? And, next time we have to undertake such a large change, what should we do differently?