Imagine if the world ran out of phone numbers. Mobile providers could issue no more smartphones, businesses could create no new call centers, and the public would be left fighting over and recycling a diminishing amount of existing phone numbers. "That's a similar situation that we're in on the Internet," says Facebook's Donn Lee, pointing out that the Web's billions of IP addresses are about to hit its max. Lee is Facebook's lead engineer on IPv6, or Internet Protocol version 6, the first new version of the Internet's addressing system in decades, which will provide trillions and trillions more unique addresses on the Web. Every time you go online or print a document at work, you're essentially dialing a unique number — called an IP address — to communicate with other devices and computer networks. It's no different than visiting a friend: You might know the name of his apartment building, but you'll need an address and zip code in order to locate it on a map. The Web works in the same way, but as the number of homes and businesses and devices connected to the Internet continues to increase exponentially, we're rapidly running out of space online. And now it's up to Internet giants such as Facebook, Google, and Yahoo to fix the problem before it's too late. The solution is simple to understand but difficult to implement. The original version of the Web's addressing system, IPv4 (Internet Protocol version 4), had a shorter range of numbers — think a phone number with only, say, ten digits — that yielded roughly 4.3 billion available addresses or combinations. With IPv6, more digits are being added to that phone number, thus "increasing the number range," Lee says. "The whole address is now 2^128, which is a huge, nearly infinite number." According to one report, IPv6 will enable 340 trillion, trillion, trillion addresses. To transition to the upgraded system, however, IPv6 requires a big push from the Web's biggest players, and on Wednesday, the first steps are being taking on World IPv6 Day, a 24-hour test to weed out any bugs and accelerate adoption, which some are calling the biggest experiment in the history of the Internet. "We're not switching to IPv6, we're becoming bilingual: We're continuing to support IPv4, but adding IPv6 compatibility," Lee says. "It's the first time it's ever been done on global scale, with such huge participation around the world." The test marks a big transition on the Web from public to private initiative. In the 1970s, DARPA began funding research that would eventually lead to the creation of the first Internet Protocol address system. By the 1980s, DARPA rolled out the Internet to the public, with the fourth revision of the address system, IPv4, as its standard. Yet now, as IPv4 is all but tapped out, the solution to the problem won't come from DARPA--the solution has fallen on the shoulders of big private tech companies. "World IPv6 Day was driven primarily by the large websites," Lee says. "To my knowledge, there was no requests to the government to migrate to IPv6." Last summer, the ball started rolling at an IPv6 conference, when Google engineer Lorenzo Colitti began discussing a solution with Lee. "Lorenzo essentially had this idea that Google couldn't turn on IPv6 by themselves, but if we could get a few of us to agree to turn it on at the same time, then we might be able to do it," Lee recalls. "I went back back to Facebook and asked whether we could do this. After we talked it over, I called up Lorenzo, and said, 'I think I can make this thing happen.' And we thought we'd probably only need to add one more company. If any one pulled out, it would fall, like a three-legged stool. We essentially went to Yahoo and said, 'Two of us are in, we need one more, can you be the third?'" Tomorrow, as the World IPv6 Day test fires up, Facebook, Google, and Yahoo — traditionally competitors in the Internet landscape — will come together with hundreds of other major organizations to pool resources for a greater cause. "IPv6 is much bigger than any one company," Lee says. "We felt that it was for the good of the Internet and future generations of the Internet." source they would most likely have to abandon domain names.