However, what I'm trying to understand is why the motivation to rapidly go from v4 to v6 only? What are the factors I'm missing in operating v4/v6 combined for some time?
Growth. Lack of IPv4 addresses will put the brakes on growth of the Internet which will have a major impact on revenue growth. Before long stock market analysts are going to be asking tough questions, and CEOs are suddenly going to see the IPv6 light. By offering pure IPv6 edge services, you can continue to grow the network unhampered by IPv4 exhaustion. For instance, offering consumer Internet connectivity using pure IPv6 from your edge router/DSLAM/termserver to the customer. If the customer sends you IPv4 packets, you drop them because you only route IPv6 for them. At the very least this will involve running some kind of proxy farm so that IPv6-only customers can still access IPv4-only Internet services. And it will also require fully functional IPv6 peering and transit agreements so that the IPv6 traffic can get to and from the IPv6 Internet effectively. You will be running a mixed v4/v6 network for the next 25 years, because IPv4 is not going away but if you refuse to add commercial IPv6 capability to your network, then you are putting the brakes on growth. Pure and simple. --Michael Dillon P.S. I think this is the real IPv6 killer app, i.e. helping the CEO keep market analysts happy and keeping the company alive through the IPv4 exhaustion crisis. A lot of telecoms companies will not survive this crisis.