After years of warnings that the Internet's predominant addressing system would run out of these numbers, the bottom of the barrel has finally been scraped. The Internet Assigned Numbers Authority (IANA) announced Thursday that it has delegated the final 300 million addresses available through version 4 of the Internet protocol (IPv4) to the five Regional Internet Registries. These RIRs will over the next few years assign these remaining addresses to new Internet-connected computers, smart phones, televisions and other devices worldwide

The distribution of IPv4's remaining addresses could be described as "one of the most important days in the Internet's history," Rod Beckstrom, president and CEO of the Internet Corporation for Assigned Names and Numbers (ICANN), said at a press conference commemorating the announcement. (ICANN operates the IANA.) "It marks far more than the transition from one Internet address protocol to another; it marks the amazingly successful growth of the Internet."

Indeed, IPv4's depletion provides some measure of the Internet's popularity, given that the protocol allowed for nearly 4.3 billion addresses. The dearth of IPv4 addresses also means that its successor, IPv6, is now thrust into the spotlight. (IPv5 was an experiment that failed to scale adequately and was subsequently abandoned.)

Internet service providers (ISPs) now need to step up and implement IPv6, says Vint Cerf, Google's Chief Internet Evangelist and a former Defense Advanced Research Projects Agency (DARPA) scientist instrumental in creating the Internet. Whereas IPv6 has been available for the past 15 years, ISPs were able to squeeze a lot of mileage out of IPv4 addresses using network address translation boxes to enable many private addresses to share a single public IP (Internet protocol) address, according to Cerf, a former ICANN chairman.

"So the ISPs didn't implement IPv6 even though the operating system vendors and router vendors did implement the protocol," Cerf says. "What is needed now is a major effort to implement the protocol in the ISP space and to test the system end to end." There are a lot of details that "have to be gotten right" for ISPs to install the operationally solid dual-stack systems necessary in the near term to support both IPv4 and IPv6, he adds.

Every device that connects to the Internet has a unique identifier generated by the IP addressing system. Since 1982 most of these have come from IPv4, which generates 32-bit addresses as four sets of numbers (each with a value between 0 and 255) separated by dots. IPv6, standardized in 1996, expands the Internet address size to 128 bits and consists of eight sets of hexadecimal digits separated by colons. IPv6 thereby offers one billion-trillion times more addresses than IPv4.

"We've all heard predictions about how in the future our refrigerators will be connected to the Internet to alert us when we're out of milk or butter, our lights will be controlled by our smart phones and our cars will be wifi hotspots on wheels," Beckstrom said. "For all that to happen, we need Internet addresses, and that means we need to speed the global adoption of IPv6."

As new devices come online, they are beginning to receive IPv6 addresses. This is likely to mean little to people buying these devices, but it is very important to businesses, social networks and other organizations trying to reach those people. Web sites whose e-mail and Web servers are configured to communicate only with IPv4 addresses cannot be accessed by IPv6 devices.

Granted, the Internet will not be significantly different next week than it was this week, Olaf Kolkman, chairman of the Internet Architecture Board (IAB), an Internet Society (ISOC) committee that performs oversight of the Internet's technical and engineering development, acknowledged at the press conference. In the long term, however, Web sites will find it difficult to support both IPv4- and IPv6-enabled networks. For this reason, Google has been supporting IPv6 since early 2008 and moved YouTube to the new protocol in February 2010. To promote the move to IPv6, the ISOC is hosting World IPv6 day on June 8, during which Facebook, Google, Yahoo, Cisco and other companies will offer their content over IPv6 for a 24-hour test period.

The amount of time it takes to assign the remaining IPv4 addresses will depend on each RIR's policies, although it is estimated that the first region to run out of addresses will be Asia-Pacific given the rapid pace at which people there are adding Internet-connected devices, Kolkman said.

IPv4 and IPv6 will need to coexist for several decades to ensure that IPv4 devices can continue to connect to the Internet for as long as they are functioning. If the transition from IPv4 to IPv6 is handled properly, the end result should be akin to Y2K—when at the turn of the millennium computer operators feared the worst but very few serious problems actually arose.