We're about to hit phase 3 of ARIN IPV4 rationing

It depends on the policies of the RIR (Regional Internet Registry) in the region where the IPs are from.

That said, if you're willing to ignore the RIR's policies (which a lot of people do) you can usually "black market" trade IP addresses, so long as you can convince your upstreams to accept them (which they usually will).

There's always been a vocal group who maintains that the IP addresses that are not visible on the internet should be returned. There are a number of problems with this argument.

Firstly, there are legitimate business reasons for having assigned, non-RFC1918 addresses that are only used internally. Interconnecting private networks belonging to different organizations that have overlapping 10.x addresses is a painful process. Sure it's possible to NAT, but given that the IP addresses are effectively an asset of the organization, why not use them? I'm not implying that this is right, but it's how the system was in the past and the cost of changing internal networking and applications could be very high.

Secondly, how do you prove that an IP address is accessible on the Internet? Many IPs do not respond to pings and I can easily set up a device that will answer for all IPs behind the corporate gateways.

I'd really hoped we'd have more support for IPv6 by now. It's funny, every so often there's a big "We need IPv6" push with lots of media coverage which is forgotten about shortly afterwards.

As far as I know there's only one ISP (Internode) here in Australia which offers IPv6.

Much of that space can't be recovered though because it's just little non-contiguous bits here and there...

Five (or ten) years ago, NAT was "the solution" – which helped and worked for longer than the doomsayers predicted.

Today, I suspect SNI will be the important stop-gap measure. There's a whole bunch of websites out there on shared hosting with dedicated IP(v4) addresses for their vhost just so they can use SSL certs for https connections. If we can ignore Windows XP users and IE6 users, SNI allows SSL certs on shared IP addresses - if I can think of a few dozen "unnecessary" IP addresses this little web development firm consumes, I suspect big hosting companies could probably find thousands or tens of thousands of similarly used IP addresses.

(Having said that, analytics still shows a startlingly high number of WinXP and IE6 users out there. It'd be interesting to see if any of them ever "convert" in ways that'd require SSL certs? I understand why the use of pirated XP and 10 year old hardware are rampant across the 3rd world, for _my_ clients that demographic is almost certainly not likely to be ecommerce customers... That's a bit of a personally skewed perspective though, there's no good argument to be made that says wikileaks/twitter/gmail users on old hard/software shouldn't get ssl protection, but I suspect high-end bed linen online shops wouldn't be hurt at all by using SNI and thereby increasing friction for IE6/WinXP users…)