Google and Facebook warn internet users of websites running slow on Wednesday, as the world's largest internet companies, including Microsoft, Google, Yahoo and Facebook, test the new global number system.

The testing is part of the global initiative to switch over to a new system called internet protocol version 6 (IPv6). Since Internet addresses on the current system are about to run out, the new protocol version is necessary and the test on Wednesday will be part of the switch over process.

As the telephone numbers have gone up from a couple of digits to 10 to 11 digits today, the internet will also move from 32 bit addresses to 128 bit addresses.

Hundreds of companies are participating in World IPv6 Day to run a test drive, billed by the Internet Society as the first “test flight” to prepare for IPv4 exhaustion and accelerate the momentum of IPv6 deployment.

Google has been among the most active proponents of migration to IPv6. Vint Cerf, one of the Internet’s founding fathers and now Google’s chief Internet evangelist, warned last November that online growth could be threatened if it is not implemented.

“The vast majority (99.95 per cent) of people will be able to access services without interruption,” Google said in a blogpost on Monday evening.

“However, as with any next-generation technology, there may be teething pains. We estimate that 0.05 per cent of systems may fail to fall back to IPv4, so some people may find Google, Facebook, Yahoo, Bing and other participating websites slow or unresponsive on World IPv6 Day.”

Users of older web browsers could find it difficult even to access many websites, said a statement.