Today is World IPv6 Launch Day. It’s the day the Internet gets some more legroom, headspace, and elbowroom too.

Internet pioneer Vint Cerf calls it the launch of the “21st century Internet,” but what’s really happening is that a number of Internet giants — Google and AT&T and Comcast and Facebook, to name a few — are turning on new gear that will help solve a very big problem.

In essence, they’ll inflate the Internet’s address book from the equivalent of a flimsy back-pocket notebook (IPv4) to a long row of libraries filled with deep ledgers (IPv6) to some big websites.

To put it in perspective, IPv4 can handle just over 4 billion addresses. IPv6 spans 340,000,000,000,000,000,000,000,000,000,000,000,000.

That doesn’t mean IPv4 is going away. None of the big Web companies are going to drop IPv4. They’re just adding IPv6 as another option. And service providers such as Akamai, AT&T, and Comcast are going to do the same thing. “The idea here is to get more and more of the Internet IPv6 enabled so people can use their IPv6 environments to get there,” says Bradner.

IPv4 will be around for a long, long time. But there’s an insatiable demand for Internet-capable devices, and soon companies are going to start leaning on that brand new IPv6 address space. And the more systems out there that support the same protocol, the easier it will be for everyone to find each other in this brand new internet.

Happy World IPv6 Launch Day.