TORONTO -- How will we know when IPv4 address space is all used up? And what will happen when that day comes?

The modern Internet has been built using IPv4 , which provides for 4.3 billion address, a supply that could run dry within the next two years. Organizations that allocate IP address space like the American Registry for Internet Numbers (ARIN) have attempted to forecast when IPv4 address space will be gone, but it's not an exact science, and there is no precise date to mark on a calendar.At the ARIN XXV policy meetings held here this week, ARIN CIO Richard Jimmerson explained how the organization expects to know when the final IPv4 address is gone.

"We will run out of IPv4 address space and the real difficult part is that there is no flag date. It's a real moving date based on demand and the amount of address space we can reclaim from organizations," Jimmerson told InternetNews.com. "If things continue they way they have, ARIN will for the very first time, sometime between the middle and end of next year, receive a request for IPv4 address space that is justified and meets the policy. However, ARIN won't have the address space. So we'll have to say no for the very first time."

Saying no to an IPv4 request will be shocking to some organizations, which is why ARIN is trying to get the word out now on the importance of moving to IPv6, Jimmerson said. The IPv6 address space, the next generation of IP addressing, provides 340 trillion trillion trillion (34 x 10 to the 38th power) Internet addresses.The first time that ARIN declines an IPv4 address request won't necessarily be the date that IPv4 is completely exhausted.

"It will be a different date for different sizes and types of organizations," Jimmerson said. "For instance there are some large national organizations that make address space requests of big blocks. They'll be the first ones to come in and we'll have to tell them we don't have as much as they want and they'll have to take a smaller block. That will be the first indication."

For other organizations requesting smaller blocks of a few thousand addresses, IPv4 may be available for a longer period of time. But eventually, ARIN will reach the point when it won't be able to fulfill even small requests, Jimmerson said

To date, the smallest address size allocation ARIN has issued is what is referred to as a /22 address block, which provides 1,024 IP addresses. Jimmerson noted that in the future, ARIN may well begin to offer smaller sized address blocks in the /24 range, which provide 256 IPv4 addresses.

Once the final IPv4 address space that ARIN has available is allocated, there will still be some extra IPv4 addresses that the organization will hold in reserve.

"We have some special addresses that we'll hold onto, according to the policy that has been set," Jimmerson said.

He explained that members of the policy community recognized a few years back that IPv4 address space was running out. They also recognized that there would soon be some organizations that would need to deploy new networks and services on IPv6 without the benefit of IPv4. As a result, the decision was made to retain some IPv4 address space so that new networks could put up their IPv4 DNS and run protocol-translation services."So the community set up a policy where we reserve a /10 of IPv4 address space from our final /8 address allocation," Jimmerson said.

A /8 block contains 16,777,214 addresses. The /10 contains 4 million addresses.

"So in the future when we do run out of IPv4 we still have that /10 set aside for organizations that just need a little bit for protocol translation or DNS," Jimmerson said.

ARIN manages IP address space allocations for the U.S., Canada and the Caribbean region. ARIN is one of five global Regional Internet Registry (RIR) organizations that in turn receive their IP allocations from the Internet Assigned Numbers Authority (IANA). The other four RIRs are the RIPE Network Coordination Centre (RIPE NCC) for Europe, the Middle East and Central Asia, the Asia-Pacific Network Information Centre (APNIC) for Asia and the Pacific region, the Latin American and Caribbean Internet Addresses Registry (LACNIC) for Latin America and the African Network Information Centre (AfriNIC) for Africa.

At this point, Jimmerson said that ARIN will still be making IPv4 address space available as long as the supply from IANA holds out.

"There is a global policy that states when the IANA free pool of IPv4 addresses gets down to five /8s remaining they will automatically take and give one of the remaining /8s to each of the five RIRs," Jimmerson said.

According to Jimmerson, there are now 20 /8s remaining in the IANA pool, which makes it likely that ARIN will get more IPv4 address space. As IPv4 address space nears exhaustion, ARIN has seen the demand slacken.

"In the ARIN region demand for IPv4 may have leveled off and slowed down in the last few years," Jimmerson said. "We have a pretty saturated market with IPv4 address space in North America."

Other areas of the world are still seeing high demand for IPv4 address space. In particular, the Asia-Pacific and Latin America regions are experiencing historically high levels of IPv4 address demand, Jimmerson said.

"For only the second time ever, LACNIC, which services all of South and Central America, issued more IPv4 address space in the first quarter of 2010 than ARIN did," he said. "I don't think it has anything to do with IPv4 depletion -- it's just that the markets are picking up down there."

Sean Michael Kerner is a senior editor at InternetNews.com, the news service of Internet.com, the network for technology professionals.