Every computer, modem, server and smartphone that connects to the Internet has a unique Internet protocol (IP) address, which enables users to 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 (232) available.

Now they’re almost gone. In the past few years Internet and Web companies have begun snapping up a new set of addresses, known as IPv6, that have 128 digits. But the companies have not made them live. That changes on June 8, when Google, Comcast and others will turn on IPv6 addresses for a 24-hour test. 

Most likely the transition will go smoothly. All but the oldest computers and phones can handle both schemes, although the IPv6 option may not be turned on. Old DSL modems or cable modems may not be compatible, says Geoff Huston, chief scientist for the Asia Pacific Network Information Center. In these cases, if you try to access an IPv6 address, you will either experience a delay or never connect. For a few years Internet companies will have to support both formats, which could slow access. “At some point, IPv6 will dominate, and everyone will be optimized for it,” Huston says. When that day will come, no one can say.