How many people are on the Internet?

How many people are on the Internet? It seems like it should be an easy on to answer, but it's deceptively simple. However, attempting to answer it illustrates many of the problems in making supposedly simple measurements.

First, the world's population is a bit over 7 billion people, so this establishes a nice upper bound. No matter what, we know that the answer lays between zero and 7 billion.

Now, every device on the Internet has a unique IP address associated to it. It should be a simple matter to count up the number of addresses and get a better estimate. However, this is the first problem: IP addresses are unique to devices, not people. Two people sharing a computer counts as one IP address and one person with a cell phone, computer, tablet, and Internet-connected TV counts as four. However, lets ignore that problem for the moment.

Most IP addresses use what's called IPv4 and currently IPv4 addresses are "exhausted"; in other words, every address has been claimed by someone. IPv6 is the newest addressing system, but its usage is fairly small, so we'll ignore it. IPv4 addresses are a 32-bit number, so the number of addresses is 2^32 = 4,294,967,296 or approximately 4.3 billion. That means that there is an upper limit of 4.3 billion devices on the Internet.

Not all IPv4 addresses are in use due to inefficiencies in the distribution system. The first users of the Internet were given large blocks of addresses for them to dole out to their users, however many of these initial blocks were extremely large. For example, Stanford University was allocated 16.7 million addresses. That's a bit of overkill for a university with less than 20,000 people on campus. They later returned the unused addresses, but these sorts of inefficiencies remain at a smaller scale. For example, many ISPs maintain smaller blocks of unused addresses so that they can give them to new customers. With this in mind, the 4.3 billion device count was an overestimate.

However, Network Address Translation (NAT) can cause underestimates. NAT allows many devices to share one IP address. Most home users have one IP address for their Internet connection and every Internet-connected device (computer, laptop, tablet, cellphone, TV, Bluray, etc) shares that IP address using NAT. So, our 4.3 billion device count is also an underestimate.

So, the number of people on the Internet is somewhere around 4 billion. Possibly more due to people sharing devices and NAT. Possibly less due to people with multiple devices and unused IP addresses.

But what got me thinking about this was a comment from a video game publisher about the number of pirated copies of their latest game. If we can't even figure out reasonably well how many people are on the Internet, how can we possibly answer more sophisticated questions like how many pirated copies of a particular game are there?