The Internet has finally run out of addresses. Well, sort of.

As we noted last year, the issue involves public IP addresses, which points to a physical computer or server that stores a website, rather than the domain names (such as geeksaresexy.net) that most human users type in.

The current system, Internet Protocol version 4 was introduced in 1981. It uses 32-bit addresses, meaning a maximum of 4,294,967,296 possible combinations. At the time it presumably seemed inconceivable there’d ever be that many devices connected to the Internet, a presumption that is now out of date.

IPv4 numbers are handed out in batches by the Internet Assigned Numbers Authority (part of ICANN, the closest thing the Internet has to a governing body) to one of five regional registries, from which they eventually end up with individual site owners.

This week the last major batch of available addresses went to the Asia Pacific registry. That left five batches and triggered a pre-arranged process by which one went to each registry immediately.

Of course, this doesn’t mean there are no addresses left: ICANN likens it to the last stock leaving a manufacturer’s warehouse, but still being in the distribution system and store shelves not being empty yet. But that day is coming: ICANN calls it only a matter of time before individual registries run out, and the BBC goes as far as to predict September 2011 for the system to be completely used up.

The next step is the adoption of IPv6, which has a billion-trillion times larger range of possible addresses. Rather cutely, ICANN only goes as far as to describe this as “virtually inexhaustible for the foreseeable future.”

With that becoming a more pressing issue, a group called the Internet Society is co-ordinating World IPv6 day on June 8. That will involve some of the largest websites such as Google and Facebook simultaneously testing a “dual-stack Internet connection” system. The idea is that connections will be set up so that all users whose equipment, such as routers and browsers, are able to cope with IPv6 will connect that way, while other users’ computers will automatically reroute to the site’s IPv4 address.

In some cases this won’t happen: either the computer won’t connect at all, or there’ll be a noticeable delay. The estimate is that 0.5% of attempted connections will fail, a proportion considered unacceptably high on a permanent basis. The idea of the test is to find out if that estimate is right, find out exactly which set-ups are connected, and maybe even give some users enough of a shock to force them to upgrade their equipment where needed.