Earlier today, someone I follow on Twitter retweeted a message from Alyssa Milano. This is what the former TV star had to say:

Less Than 1 Year Until The Internet Runs Out of Addresses

This wasn't the first time someone said that we were going to run out of IPv4 addresses soon. Or, to stick to the TV theme: all of this has happened before, and all of this will happen again.

It all started with the Internet Engineering Task Force in the early 1990s. Reportedly, it was determined that we'd run out of addresses in 2005. Two years before that apocalyptic year, on September 4, 2003, I was sitting in the Grand Ballroom of the Krasnapolsky Hotel in the center of Amsterdam for the last day of sessions of the RIPE-46 meeting. (The RIPE NCC is the organization that gives out IP addresses in Europe and some other regions.) Geoff Huston gave a presentation titled "IPv4 Address Lifetime Expectancy Revisited" where he showed the trends in IP address deployment, and used a simple model to extrapolate these trends to predict the moment the last IP address would be used up. According to the then available data and the model, that last IPv4 address would be put into actual use somewhere between 2038 and 2045—no, that's not military time for later tonight, but sometime between Y2K38 and 100 years after the end of World War II.

So we went from "two years left" to "35 years left" in less than an afternoon. (Of course by 2003 everyone had realized we wouldn't run out of addresses in 2005 and there was widespread skepticism about Huston's results, but still.)

Two years later, in 2005, Geoff Huston was back in Amsterdam for the RIPE-51 meeting with a new presentation, also titled "IPv4 Address Lifetime Expectancy Revisited." In the intervening two years, he had adjusted his model and chose the moment the first of the five Regional Internet Registries (RIRs) runs out of addresses as the moment of truth, rather than the moment the last ISP puts the last address into actual use. This, combined with the sharp increase in IPv4 address use in 2004 and 2005, resulted in two new dates: the IANA pool would run out on August 5, 2012 and the first RIR would run out of addresses on May 2, 2014. Of course that didn't quite unring the bell from two years earlier—fool me once...

Since that time, Huston's automatically generated predictions have climbed up to (at least) 2016 and then gone back down again, with the current predictions estimating the day the Internet Assigned Numbers Authority (IANA) gives out its last block of 16,777,216 addresses to one of the RIRs to be July 1, 2011, and the day the last IPv4 address is given out by an RIR as January 20, 2012.

It's this first projected date that is getting all the attention right now. There are 221 usable blocks of 16.8 million addresses ("/8s") in the IANA global pool. 205 have been given out to RIRs or large organizations such MIT, Apple, HP, and especially the US military. That leaves 16. The IANA and the RIRs have agreed that IANA can give out two /8s at a time, and that each of the RIRs gets one of the last five. So depending on how exactly they apply this rule, once the IANA pool reaches 6 or 7 /8s, those last blocks can evaporate overnight. Given that we've been using up about one /8 each month, a year until this happens seems a reasonable estimate. John Curran, president and CEO of ARIN, tells Ars that it could happen much faster, however.

"The rate of allocation from the IANA central pool has picked up; we've allocated 10 /8s since January 1, and that's a run rate nearly twice that of previous years," Curran told Ars. "If the rate of the last six months were to continue for the remainder of the year, we could actually be at 5 /8s by year-end."

That could happen, but allocation of /8s to the RIRs has happened in bursts in the past, so the increase in allocation rate earlier this year could also be completely meaningless. The interesting part is what comes after the IANA global pool has been depleted. AfriNIC (Africa) and LACNIC (Latin America and the Caribbean) use relatively few addresses per year, so they'll be able to draw from their own pools for some years to come. Although APNIC in the Asia-Pacific region burns through addresses the fastest, the RIRs keep their own pools at a size that is sufficient for nine to 18 months of operation. So any of the three large RIRs could be the first to run out. And within two years, all three will. But this could happen a lot faster if there is a "run on the bank" as ISPs and other address users put in their final requests post haste.

In the meantime, the RIRs are sending their people to the far reaches of the planet to spread the gospel of IPv6. With Facebook following Google/YouTube as a top Web property starting to roll out IPv6 support and Apple adding IPv6 to iOS 4, IPv6 is definitely gaining momentum. While being IPv6-ready isn't going to make anyone immune to the problems that will certainly arise when we run out of IPv4 address space, it's a fair bet that those with IPv6 will have an easier ride than those without.

More on this next week when we report from the IETF meeting in Maastricht, the Netherlands.