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The original pool of internet addresses has officially run dry.

The last five blocks of the IP Version 4 addresses have been handed over to the regional bodies that distribute them.

Those five blocks, called /8s and which contain 16 million addresses each, are expected to be completely depleted by September 2011.

The move to the new addressing scheme, IP version 6, is under way but could take years to complete.

"This is one of the most important days in the internet's history," said Rod Beckstrom, head of net overseer Icann at a press conference called to mark the handing over of the final five blocks.

"It is a point that the founders of the internet thought would occur far in the future," he said. "It gives us an opportunity to shift to an internet protocol that offers a pool so large that it is difficult even to imagine."

IPv6 has a pool of addresses a billion, trillion times larger than the 4.3 billion that IPv4 can support.

While that pool of 4.3 billion addresses was seen as plenty when the net was getting going, its recent growth has seen it get used up very quickly.

The shift to IPv6 was needed, he said, to support the continuing growth of the net and its greater use by all kinds of connected devices.

"The future of the internet and the innovation it fosters lies with IPv6," said Mr Beckstrom.

Despite the imminent exhaustion of the IPv4 pool, few ISPs, companies, academic organisations and others have made the switch. A World IPv6 Day is being planned for 8 June that will give governments, companies and others the chance to test out the technology.

Cisco, Verizon, Yahoo, Google, Facebook and many others have committed to testing IPv6 on 8 June.

In the UK, the switch to the new addressing scheme might take years, said Philip Sheldrake, head of 6UK, an organisation set up to advise companies how to make the move.

Most firms would upgrade equipment, routers, hubs and modems, as part of their "normal replenishment cycle", he said.

Equipment that is going to be in use for years before being replaced could be fitted with "dual stack" systems that can handle both IPv4 and IPv6 addresses

Some ISPs and others may take a more aggressive approach to the switch, said Mr Sheldrake.

"There are automated approaches that involve some remote updating of firmware in equipment," he said.

"The internet does not break when IPv4 is exhausted," said Mr Sheldrake explaining why this long term shift was feasible.

"What we are looking at here is that some parts of the world that have less IPv4 will be more on the front foot of adopting v6 than the UK because we have some good v4 space," he said.