Every computer, modem, server and smart phone that connects to the Internet has a unique Internet protocol (IP) address, so users can find it. The address format, known as IPv4, was standardized in 1977 as a 32-digit binary number, making a then-seemingly unlimited 4.3 billion addresses (2^32) available.

They're all used up.

How? Well, for decades the Internet Assigned Numbers Authority has doled out blocks of IPv4 addresses, as needed, to five Regional Internet Registries around the world, which then assign addresses to users one by one. In February, however, the authority gave each registry one final block of 16 million addresses. The regions are burning through them now, and one region—Asia–Pacific—has already hit zero.

Since 1999 the authority has offered blocks of newer IPv6 addresses that are 128 digits long, resulting in an unimaginable 340 undecillion possible addresses (that's 340 followed by 36 zeroes). But until 2008 or so, few organizations bothered to ask their registries for them. Now Internet carriers, Web companies and Internet service providers (ISP) large and small are sucking up IPv6 addresses for their old and new machines, but in most cases without making the IPv6 addresses live. So a moment of truth has arrived—Google, Yahoo, Facebook, Comcast and others are turning IPv6 on for 24 hours to see what happens. The exact start time for Global IPv6 Day varies depending upon location (in some places, the test actually began on Tuesday). To find out when testing begins in your region, consult this Web site.

All but the oldest computers and phones have been configured to handle both schemes, but "home gateways—the DSL modems or cable modems—may not be," says Geoff Huston, chief scientist for the Asia–Pacific regional registry. And the IPv6 option in your computer or phone may not be turned on. In these cases, if you try to access an IPv6 address on June 8, you will either experience a delay of up to 75 seconds, as your system finds its way to the IPv4 address for the site you're trying to reach—or you may just never connect.

"We're hoping that on IPv6 World Day we'll see v6 traffic go up, so we'll have a better idea of how many users are capable of using IPv6," says Timothy Winters, a senior manager at the University of New Hampshire InterOperability Laboratory (UNH-IOL), which offers broadband service–providers and network equipment–makers the facilities and expertise for testing their products. "Right now, when users go to sites like Google or Facebook we're not getting good statistics because users can only get there via IPv4."

On the flip side, testers also expect problems with IPv6 to surface during this daylong stress test. "Some things are going to break, and when that happens it gives us some idea of how many users can't use v6," Winters says. The best evidence for determining just how many people's systems are not IPv6-compatible will come from calls to ISPs complaining of poor service, or none at all. "If Google or Facebook did this testing on their own, you'd have all of these Internet users calling their ISPs, but the ISPs wouldn't know the IPv6 test might be behind their customers' problems," he adds. "[June 8] is a nice day for us to try out v6 and then we can shut it down and fix all of the problems that may arise."

The parties undertaking the test have scattered diagnostics across the Internet, and will be able to see if 10 or 1 or 0.1 percent of users, for example, experience problems. They will also be able to tell if the problem is your machine, their machines, your service provider or some other node in the Internet. Even a small portion of disconnects is a big deal, however; 1 percent of the Net's two billion users represents 20 million people.

As the last few IPv4 addresses are expended, region by region, over the next few years, new machines will be forced to have only IPv6 addresses. Old IPv4-only machines may not be able to find the new IPv6 machines. Internet operators will therefore have to run systems in both formats for at least several transition years, and perhaps longer, which will add cost and could slow access. "At some point, IPv6 will dominate, and everyone will optimize for it," Huston says. "When that will be, I can't say. We're a large, diverse industry, and no one is in charge."