Delays in transition to IPv6 system mean some households and businesses will face connection problems in coming months

The internet is full. Or, to be more precise, it has run out of new internet addresses – having used up almost 4 billion since being set up 40-odd years ago.

The Internet Address and Naming Agency (IANA), which doles out blocks of new internet addresses (consisting of machine-addressable numbers rather like a phone book), said on Tuesday morning that it will allocate the last seven remaining blocks of addresses this month, including three to the overall internet registrar for Asia and the Pacific.

This could mean that in a year's time you may hear about a new site – yet when you type its address into a web browser or click a link to it on a web page, your computer simply won't connect to it because it will use an addressing system entirely different to the one used before.

It could even get worse than that, according to James Blessing, a member of the board of the UK's Internet Service Providers Association (ISPA). "You might find that you can't get online unless someone else goes offline," he said. "It would be like the internet before broadband, when everything was on dial-up modems, and if too many people were dialling in then you couldn't get connected."

The problem has been exacerbated, experts agree, because ISPs, governments and companies that make the routers used in households and businesses have ignored the problem until the last moment.

Nor is there any way to estimate the costs involved in changing, because it depends on how well set up each ISP and each household is.

Alex Pawlik, chief executive of the network coordination centre RIPE (Réseaux IP Européens), which allocates internet addresses in Europe, said: "I think in about six months' time people will be waking up and will have to explain to their boss why they haven't done anything about it."

The old system, called "IPv4" – for internet protocol version 4 – has stood the internet in good stead since its inception. It could support a total of 4.3bn internet addresses, and many more by using certain networking tricks.

But with the explosion in the number of devices, particularly mobile phones, connecting to the net, more and more addresses are needed.

A technical solution, called IPv6, already exists, and has been waiting in the wings for more than a decade. Rather than the 4.3bn limit of IPv4, IPv6 would allow about 340 trillion devices to be on the internet at once – or, as Blessing puts it, "everyone could have their own internet".

RIPE is preparing to hand out the last-ever block of addresses that will work with existing systems in the next few days to internet registrars. Those will be sold by registrars for new websites or internet services in the coming months. And once those are gone, RIPE and the world's other six regional registrars will only be able to hand out IPv6 addresses.

However, an IPv6 address is in effect unreadable by equipment set up to handle IPv4 addresses, which comprises pretty much every piece of computer and network equipment on sale, apart from newer computers running Microsoft's Windows 7 or Apple's Mac OS X, and most smartphones. Older system such as PCs running the ten-year-old Windows XP – still in widespread use – and the broadband modems used in households cannot understand IPv6 addresses without special configuration that would defeat most users.

But nobody has been willing to spend the time and money to implement IPv6 until the end of the old system has become imminent, says Blessing.

"People have been saying that the IPv4 address space hasn't run out yet for some time," Blessing says. "It's like oil wells. This is the equivalent of the oil wells drying up, and us not being able to find any new ones. We know there are multiple alternatives like electricity or biofuels, but as long as we have oil we'll use that. When RIPE runs out of new IPv4 addresses, some time this year, that's like the oil wells running dry."

Philip Sheldrake, who runs the advocacy group 6uk, says that only 1% of the traffic now passing through the net uses IPv6 addressing. "It's not like the millennium bug, when there was a clear date when everything had to be done," he said. "Everything ought to continue working as before. But gradually, we are all going to have to move to v6. After all, there are only 4.3bn addresses in IPv4 – and there are six billion people on the planet."

Will this affect you and what can you do about it?

What does an IP address look like?

It's a collection of four "dotted quads" of numbers between 0 and 255, such as 77.91.248.30. That is an IPv4 address: each of the numbers is eight binary bits long, and there are four of them.

Is that the same as a web address like theguardian.com?

It is to a machine once the human-readable bit has been turned into something machine-readable by another machine called a domain name server, which does the words-to-numbers translation.

What does an IPv6 address look like?

It's usually written using two pairs of four dotted quads in hexadecimal (base 16, where the numbers 10-15 are represented by the letters a-f). So an example would be 2001:db8:1f70::999:de8:7648:6e8.

What's the problem, then?

Probably your computer, and almost certainly your household router, if confronted with an IPv6 address will think that an error has occurred, and ignore it. It would be like trying to deliver post when you don't speak the language.

When will IPv6 come in?

It has already started to be used: some sites, including Facebook, have started experimenting with it while retaining their IPv4 address. But in about 12 to 18 months, sites will start to only have IPv6 addresses.

How can I fix this?

You can't. The onus is on ISPs and router manufacturers to provide ways around the problem, including upgrades to their services and hardware. You might have to upgrade your computer's operating system, and you might have to buy a new router.