Every computer, phone, and gadget that connects to the Internet has what’s called an Internet Protocol address, or IP address—a kind of numerical name tag for every device online. And the Internet is rapidly running out of the most commonly used type of IP address, known as IPv4. Today, the American Registry for Internet Numbers (ARIN), the organization responsible for issuing IP addresses in North America, said that it has run out of freely available IPv4 addresses.

That won’t affect normal Internet users, but it will put more pressure on Internet service providers, software companies, and large organizations to accelerate their migration to IPv4’s successor, IPv6.

Yes, this news may sound familiar. WIRED reported back in 2011 that the Internet had run out of IP addresses, or more specifically, that an organization called Internet Assigned Numbers Authority (IANA) had run out of IPv4 addresses. Basically, IANA hands out blocks of IP addresses to regional organizations like ARIN and its counterparts around the world. So even after IANA ran out, many IPv4 addressees were still available. But now the regional organizations are running out, as well.

ARIN president John Curran explains that the organization isn’t entirely out of IPv4 addresses. Some are set aside for specific purposes, such as the exchange sites where connections between different Internet service providers’ networks meet. But providers that want new IP addresses will have to settle for IPv6 numbers unless old, unused IPv4 addresses are returned to the organization. ARIN has a waiting list for companies that want to get their hands on some of these recycled numbers.

Technologists have known for years that we would run out of IPv4 addresses, which is why the IPv6 standard was created in the late 1990s. While IPv4 was limited to just about 4 billion addresses, IPv6 will provide 340 undecillion addresss (a one followed by thirty-six zeroes). That’s —enough to give 5×1028 addresses to every single person on the planet. And it’s already supported by all major operating systems.

The problem is that IPv4 and IPv6 aren’t entirely compatible. If you’re on an IPv6 network, you can’t browse a site running on a web server that uses only IPv4—such as WIRED’s site—without some sort of compatibility layer in between. Fortunately, Internet service providers have been working hard to update their infrastructure and support both standards.

Curran says Internet providers are doing a good job of the transition so far. In fact, most smart phones are already using IPv6, he says, and most people never notice. Just today Comcast, the largest Internet provider in the US, said its entire network now supports both IPv4 and IPv6.