If you had asked John Curran, head of the American Registry of Internet Numbers (ARIN), three years ago to pinpoint the date at which we would run out of IP address space under IPv4, he would have said 2025.

Two years ago, his answer would have been 2017.

He recently told an audience at the Burton Group's Catalyst Conference that the doomsday date is now 2011. That's not a scare tactic -- it's a fact based on the rapidly increasing pace at which blocks of addresses are being allocated, according to Curran.

He said that 68 percent of all available IPv4 addresses are gone and only 19 percent are available. The remaining 13 percent fall into a strange category of being unavailable, which means blocks of addresses that belong to a large company - a university or an ISP - and are not used.

Agencies such as ARIN encourage the owners of these addresses to return them to the pool - he pointed out that Stanford University, for example, recently did just that with its excess addresses - but Internet governing bodies can only request; they have no power to take back those unused addresses.

Curran said that by 2010, agencies such as ARIN will run out of addresses to dole out. In other words, there will be "no more blocks in the central pool." ISPs will still have enough addresses on hand to meet customer demand for a year after that, but by 2011, ISPs will run out of their inventory.

This means that unless ISPs and enterprises start moving to IPv6, those ISPs will be unable to sign on new customers and enterprises will be unable, for example, to build out new networks.

Curran emphasised that the situation has not reached a crisis stage, but he said that companies and ISPs need to start now to do the configuration work needed to upgrade to IPv6. `It's inevitable,' he said.

Curran said cost estimates for an IPv6 upgrade are hard to come by, but he said ISPs will need to invest a significant amount of money to accomplish a move from IPv4 to IPv6. Clearly, the ISPs have a financial incentive to make the move -- after all, their entire business model depends on signing on new customers.

For enterprises, Curran said that companies will need to initially upgrade their Web servers and e-mail servers to IPv6 so that customers and business partners who are on IPv6 can communicate over IPv6. "People are already trying to reach your Web servers via IPv6," he said.

If companies don't upgrade, "at some point customers will not be able to reach you," said Curran. He added that companies will probably be able to keep using IPv4 internally for some time.

But everyone needs to put the move to IPv6 on their three-year planning horizon. Curran said that network devices are already IPv6 enabled, so there's no real capital expense. The task is to do the configuration work required to make the switch.