4,294,967,296

In the mid-1970s, Vint Cerf and Bob Kahn were designing a new way for computer networks to talk to each other. It would become the basic language of the internet, known as TCP/IP.

One question kept coming up: how long should an address be?

Longer addresses meant room for more machines, but they also cost precious memory and bandwidth on the computers of the day. Engineers argued back and forth.

In 1977, Cerf settled it. Addresses would be 32 binary digits long.

That gave about four billion possible addresses - at a time when the whole network connected a few hundred computers in universities, labs and military sites.

Four billion seemed like more than the world would ever need. After all, this was a research project.

4,294,967,296 sounds enormous.

But there are now more than eight billion people on Earth. If every person got one address, there would not be enough to go round - only about one address for every two people.

And people are not the only things that want to be online. Phones, laptops, tablets, televisions, games consoles, printers, doorbells, cameras, cars, watches, servers, fridges and lightbulbs all want addresses too.

Not every one of the four billion can even be used. Large blocks were set aside from the beginning for special purposes: private home networks, testing, experiments and more.

The "absurdly generous" number was always going to run out. The only question was when.

For decades, internet addresses were handed out in big blocks by a central body, the Internet Assigned Numbers Authority (IANA), to regional registries around the world, which passed them on to internet providers, universities and companies.

In the early days, some organisations were given blocks of more than 16 million addresses each. Nobody worried. There seemed to be plenty.

Then came the web, then smartphones, then everything else.

On 3 February 2011, at a ceremony in Miami, IANA handed out its final five large blocks - one to each of the world's five regional registries.

The central pool was empty.

The regional registries ran dry one by one over the years that followed: Asia-Pacific in 2011, Latin America in 2014, North America in 2015, Africa in 2017, and Europe in 2019.

The internet had officially run out of new addresses.

So how are billions of devices still online, if there are not enough addresses?

The main trick is called NAT - network address translation.

Think of a large office building with one street address. The post arrives at the front desk, and staff pass each letter to the right person inside. Outsiders see one address. Inside, hundreds of people share it.

Your home router does the same thing. Every phone, laptop and television in your house gets a private address that only works inside your home, while the whole household shares one public address on the internet.

Internet providers now often do the same thing on a much bigger scale, putting thousands of customers behind shared addresses.

It works. But it is a patch, not a cure.

4,294,967,296 is a number that almost everyone has used without ever seeing it.

It sat underneath the growth of the web, of email, of smartphones and of streaming. It was the size of the world's address book for the most important decades of the internet.

And its story is a very human one. A sensible choice, made for a small experiment by people who could not have imagined smartphones, grew into the backbone of a planet.

The experiment did not end. It simply became the world.