Facebook, Google, Yahoo and hundreds of the world's largest websites are now testing the enormously expanded internet address system, known as IPv6, in preparation for the time – expected within the next year – when it becomes impossible to add new sites to the web using the existing system, called IPv4.

The biggest problem though in implementing the new system lies inside people's homes – where barely any of the broadband routers now in use can handle the new system.

In the UK, the communications minister Ed Vaizey is marking World IPv6 Day with an industry summit to discuss progress on switching to the new protocol. Vaizey said: "We must ensure we create the conditions that allow the internet to continue to grow and for companies to produce fresh innovations. IPv6 is key to the continued growth of the internet. It is essential industry deals with the switchover before there is an impact on the internet."

Earlier this year the Google chairman, Eric Schmidt, said that the IPv4 address exhaustion was "one of the urgent problems" that the industry needed to tackle.

The "test flight" day is being used to motivate organisations throughout the internet industry – including internet service providers (ISPs), router manufacturers, computers running Microsoft Windows and Apple Mac OSX and web businesses – to get ready for the time when the IPv4 "address space" runs out.

Only 5,375 of the top 1m web sites by popularity have IPv6 connections, according to Atoom Net. The amount of IPv6 traffic has spiked since Tuesday as the test began, according to the website provided by Akamai recording it.

Sites on the web are connected using "internet protocol" addresses, which work rather like telephone numbers. The IPv4 system, introduced in January 1980, uses "dotted quad" addresses such as 127.255.255.0.255 77.91.248.30 to allocate locations to sites. That allows a theoretical maximum of 4.2bn connected devices on the network, though various workarounds have allowed that to be greatly expanded.

Once there are no more IPv4 addresses, new sites will only be able to join the net via an IPv6 address. That will mean people using IPv4-only equipment will not be able to reach the site. They could type the URL into their browser, but the address that it translates into will be unreadable by their computer and/or router.

The IPv4 address space has almost completely run out. The last blocks were allocated in Europe earlier this year. The growing shortage of available addresses that can be reached by existing systems has led to concerns that there will be a crunch in 2012 around big events, particularly the London Olympics, as a huge number of devices are attached all at once to the net.

IPv6 allows 3.4 x 10^38 (340 trillion trillion trillion) internet addresses, which is enough for every IPv4 address to have its own internet of IPv4 addresses. That should mean that once a shift to IPv6 is made, the world – and even other planets – will never run out. It offers a large enough address space for everyone to have their own internet space.

The problem for users, ISPs and webmasters is that IPv4 addresses cannot be translated directly into IPv6 addresses. Though an IPv4 address may point to the same site as an IPv6 address, the two use different addressing methods.

The key obstacle lies with the home routers used by millions of users, which may not be able to handle IPv6 communication. It may be possible to update their software, but the scale of the problem could mean that users or ISPs will have to carry that out manually.

In the UK, the government has supported the creation of 6UK, a not-for-profit organisation that aims to help smooth the transition to IPv6.

Abour 0.05% of internet users may have problems reaching sites which are using the system – but with more than a billion people online, that could run to tens of millions. Microsoft has offered a fix for affected Windows users.

People can test their readiness for IPv6 at the IPv6 test site.

Piers Daniell, of Fluidata, an independent connectivity company which has been working on the changeover in the UK, said: "What's going to be really interesting is to see how companies and ISPs will work with IPv6 on a daily basis. Getting websites to be viewed over IPv4 and IPv6 is proving problematic for some. Indeed, much of the rationale for World IPv6 Day is driving companies to combine their IPv4/IPv6 websites. What most companies don't realise is that their website, for a time, will need to be seen by both. This means that companies need to almost dual-stack their sites, but this is problematic and so the focus must be on solving the issue on a wider scale. Even today most hardware shipped by ISPs to providers still doesn't cater for IPv6 or will only once a software patch is applied."

• Listen also to the Tech Weekly podcast from February on IPv4 and IPv6.



Updated to correct the example of an iPv4 dotted quad.