For some time now, we've been writing about how IPv4 addresses are running out. Very soon, ARIN, the American Registry for Internet Numbers, which distributes IP addresses in North America, will have to say "no" to a member requesting IPv4 addresses, even though they qualify for getting them.

In Asia, Europe, and Latin America/the Caribbean, the local Regional Internet Registries handed out IPv4 addresses in accordance with the governing policies until they reached a "final /8" or "final /10" limit of 16,777,216 or 4,194,304 in 2011, 2012, and 2014, respectively. At that point, each member (mostly Internet Service Providers) got to request one last block of 1,024 addresses.

ARIN has adopted a slightly different approach: rather than reserve a final block of addresses that are distributed under different rules, ARIN attacked the depletion of the IPv4 address space in four phases, where each phase increased the scrutiny given to requests. We're currently in phase four, and so far, ARIN has been able to meet all qualifying requests. But at this time, the largest block of IPv4 addresses in ARIN's vaults is a /11, or 2(32-11) = 2.09 million addresses. Although requests for such large blocks don't come in every day, the largest ISPs in North America do request blocks of this size at somewhat regular intervals. Soon, that last /11 will be gone. Then there's a /12 (1.04 million addresses), a /17 (131 thousand addresses), and a /20 (4,096 addresses). After that, it's basically scraps, with only /21s (2,048 addresses) and smaller left.

"In the coming weeks, for the first time in history, an organization will come in and request IPv4 address space and qualify, but we won't have it in our inventory to fulfill the request." Richard Jimmerson, ARIN's CIO, who currently oversees ARIN's registration services, told Ars. "We've been in phase four of the countdown plan for a year now. Currently, all requests are evaluated by two team members on a first-in, first-out basis. Soon there may be two organizations that have requested /11s. One will be approved first and get our last /11. The other will potentially qualify also and then go on the waiting list, or they'll have to choose a smaller block size."

"Over the years, some IPv4 address space has been returned to the system,"Jimmerson said. "A lot of that has gone back to the IANA global pool. IANA has been redistributing that IPv4 space back equally to the five RIRs. It is possible still that IPv4 address space will be returned or reclaimed, for instance, when address holders go out of business or don't pay their fees. We didn't expect to see that anymore with the transfer system in place, but it's still happening. This address space will be used to satisfy requests on the waiting list."

There is some potential to apply game theory to the waiting list: ask for a large address block and you may not get anything, or you may be waiting for a long time. Ask for a small block and you'll get it. So it's helpful to know the size of the requests that are on the waiting list. But for now, ARIN won't be publishing the contents of the waiting list.

However, organizations that are transitioning to IPv6, the successor to IPv4 with its near-infinite address space, may request a small block of IPv4 addresses (16 to 256 addresses) to be used with transition technologies—if they haven't received any IPv4 address space in the last six months and don't currently hold any that could be used for this. ARIN has set aside a separate /10 block (4.19 million addresses) for this purpose.

"Approximately 50 percent of ARIN's members have IPv6 address space registered,"Jimmerson said. "Our message to the community is to think about IPv6 first. There's a limited supply of IPv4. Please check on the ARIN website; it's still possible to request IPv4 address space, but only for a very limited time. There's also the address space for transition purposes. But organizations that have an ongoing need for IPv4 addresses should look into the ability to transfer IPv4 addresses. We do expect transfers from the ARIN region toward the RIPE region, especially the Middle East, to pick up as soon as the policy that allows for this goes into effect due to pent-up demand. ARIN already conducts transfers to the APNIC region."

In the meantime, IPv6 deployment has been increasing slowly but steadily: Google's IPv6 measurements show that currently, a little over five percent of Google's users world-wide run the new version of the IP protocol, while during weekends we're starting to get close to seven percent. The US is now the second country in the world with IPv6 deployment at nearly 15 percent. Only Belgium has more IPv6 at 30 percent. Of course that means that the other 85 percent of Americans (well, their ISPs) may be occupying a spot on ARIN's waiting list in the near future—along with the 99.5 percent of Canadians that are still IPv4-only.