IPv6? No thanks
>> I've always wondered why haven't we already done the "Great Leap Forward" into IPv6
IMO, it's because the Internet isn't driven by people saying "wouldn't it be nice if we had longer IP addresses?" It's driven by businesses who say "how much does this IT project cost, and what benefits does it generate?"
The cost of IPv6 deployment is high. Remember that you have to upgrade not just your routers and your operating systems, but all your networked application software too.
And in return, how many businesses would see any commercial benefit (i.e. increased revenue or reduced cost)? I think few, if any.
>> IPv6 brings QoS, better routing and improved security
Sorry, but none of the above.
Perhaps people think that IPv6 brings security because it mandates IPSEC implementation. But you can run IPSEC on IPv4 too. The fact that there is no acceptable trust model for distribution of IPSEC keys (DNSSEC? Hah!) means that neither is useful for anything other than VPNs.
QoS? Where did that idea come from?
Better routing? IPv6 routing is the same as IPv4, apart from the longer prefixes.
In theory it was supposed to be easy to renumber your network in IPv6, to make it easier to change provider and maintain aggregation. In practice, it's no easier than IPv4. As anyone who's renumbered a network knows, changing your *interface* configuration is the easy part; changing all your interdependent *application* configurations is the hard part.
IPv6 doesn't offer any solution to the multihoming problem either. Registry policies on PI space are irrelevant. If businesses need to multihome, then they will buy the service, and so ISPs will make the necessary route announcements, leading to the same explosion of routing tables as IPv4 has now.
Where are the benefits? There is only one, and that's the availability of more addresses. (Of course, if we started with a brand-new IPv4 Internet without all those legacy classful allocations, we'd be fine too, but that's a side issue)
So let's suppose the day comes along when an ISP goes to a registry and is told there are no more addresses available, period. The ISP will then have two alternatives:
(1) Give their customers private IPv4 addresses behind a NAT firewall. For those few users who want to receive incoming connections, have application-level proxies (e.g. SMTP, HTTP)
(2) Give their customers IPv6 addresses, and also set up a NAT firewall for them to be able to access the IPv4 Internet, which is where all the content is anyway.
Solution (1) works today. It can even be sold as a "security" benefit to customers, since the user will be behind the ISP's firewall. The majority of users won't see any difference.
Solution (2) is a pain to implement for both the ISP and the customer. ISPs work on tight margins. Do they want the support overhead of getting all their customers to upgrade and reconfigure their endpoints to IPv6? (Again, including all application software?)
In the "no more IPv4" scenario, gamers and peer-to-peer filesharers may be persuaded to switch to IPv6, as they would see a benefit. But the majority will be happy with NAT.