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Leading internet firms have set 6 June as the World IPv6 launch day.

IPv6 is the new net address system that replaces the current protocol IPv4, which is about to run out of spaces to allocate.

Web companies participating in the event have pledged to enable IPv6 on their main websites from that date.

The Internet Society, which made the announcement, said the day represented "a major milestone" in the deployment of the standard.

Facebook, Google, Microsoft Bing and Yahoo are the inaugural web firms involved.

Every device connected to the internet is assigned an internet protocol (IP) address, which is a string of numbers that allows other devices to recognise where data comes from or should be sent to.

The IPv4 system has approximately four billion IP addresses.

The growth in the number of smartphones, PCs and other web devices and services meant that net regulator Icann had already handed out its last IPv4 sets to regional registries.

At the time it said businesses needed to start preparing themselves for a switch to the IPv6 standard, which offers more than 340 trillion trillion trillion addresses.

To put that number in context´óÏó´«Ã½ Future Media bloggedlast year that if "every man, woman and child on Earth had a billion devices each with an IPv6 address, you haven't even come close to scratching the surface of the number of addresses available".

Experts say the new system should ensure there are enough addresses for the foreseeable future.

IPv6 is incompatible with IPv4, so the transition has required old hardware to be replaced or updated.

Internet service providers (ISP) taking part have promised that by the launch date they will have enabled at least 1% of their fixed line subscribers to visit IPv6-enabled websites. The ISPs involved include the US firms AT&T and Comcast, and the Dutch firm XS4all.

The home networking equipment manufacturers Cisco and D-Link say they aim to enable IPv6 on all their home router products by the date.

And Akami and Limelight - two firms that help improve third parties' delivery of content over the net - have also promised to allow their customers to join the list of firms participating in the scheme by enabling the new protocol throughout their infrastructure.

Amsterdam-based RIPE NCC, which allocates IP addresses in Europe, the Middle East and parts of Asia, said: "Operational experience and measurements on World IPv6 Launch will help content providers and ISPs to identify and rectify any potential problems with delivering services."

Facebook's vice president of infrastructure engineering, Jay Parikh, added: "Last year's industry-wide test of IPv6 successfully showed that the global adoption of IPv6 is the best way to keep web devices communicating in the future.

"Permanently enabling IPv6 is vital to keeping the internet open and ensuring people stay connected online as the number of web users and devices continue to grow."