Google and Facebook have warned Internet users to expect "teething pains" on Wednesday as leading Internet companies test drive a new global numbering system for cyberspace.
Websites may be slow or hard to reach as more than 400 of the world largest Internet companies, including Microsoft, Google, Yahoo and Facebook, test the new system.
The switch to a new system - known as Internet Protocol version 6 (IPv6) - is essential because Internet addresses on the current system are about to run out.
The last of the addresses were allocated in February. In order to accommodate the millions of people coming on to the Internet in emerging markets and the new mobile phones and tablet devices needing online connections, the number of addresses has to be increased.
Much as telephone numbers have gone from a couple of digits in the early days of telephony to strings of 10 or 11 numbers today, the Internet will move from 32-bit addresses to 128-bit addresses. This will allow 4 billion times more Internet identifiers to be created than exist today.
However, compatibility between the two numbering systems is far from straightforward. A home computer connecting via IPv4 will not be able to connect to a website running on IPv6. Experts warn that if the two systems are not connected, millions of people in fast-growing Internet markets such as China and India could struggle to access websites in the United States and Europe, and vice-versa.
When Heise Online, one of Germany's biggest news sites, experimented with putting its pages on IPv6 last year, some users were unable to access the site. People using older versions of Internet browsers and the Mac operating system struggled to load up the pages, although Heise concluded that the number of problems was "smaller than previously feared."
A larger-scale test will run on Wednesday, with hundreds of companies participating in World IPv6 Day, billed by the Internet Society as the first "test flight" to prepare for IPv4 exhaustion and accelerate the momentum of IPv6 deployment.
Participants include the world's biggest websites, as well as Akamai, Cisco, Huawei and Limelight, who provide technology and infrastructure in the Internet's backbone.
"The vast majority (99.95 per cent) of people will be able to access services without interruption," Google said in a blog post 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 or "misconfigured" home networking equipment could find that they have difficulty accessing many sites.
"Since the best way to find bugs in your services is to hammer on them yourself, Google employees have been operating in 'World IPv6 Day mode' for several months now," Google said.
"IPv6 is used extensively in many large networks, but has never been enabled at a global scale," said Facebook in its blog post on the test. "World IPv6 Day will allow us to better understand how our infrastructure and code perform under IPv6 while minimizing impact on our users."
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.
Google will be moving its search, e-mail and YouTube video services over to IPv6 for the day on Wednesday, starting from early morning in Asia.