The problem boils down to a real-estate shortage. Specifically, there’s a scarcity of Internet addresses, the critical and unique numbers that are used to distinguish one computer from another. ... If you do the math, you quickly find out that system offers roughly 4.3 billion possible I.P. addresses. That’s a lot of addresses. But we’ve also been hooking up a lot of things to the Internet, all over the world. ... For companies in the U.S., IP addresses are parceled out by the American Registry for Internet Numbers. And the end is near, says John Curran, the CEO of ARIN. “We expect that sometime in the first half of next year, the central pool will run out,” he says.