IP addresses

An IP address works like the return address on a letter. Every message you send across the internet carries one, so the reply knows where to come back to. Without it, no page could ever reach your screen.

Usually it is your network's address, not yours

At home, your router (the box that runs your Wi-Fi) usually shares one public IP address between every device in the house. To a website, your laptop, your phone and your housemate's tablet can all look like they come from the same address.

On a phone network it is often even more shared: many customers can appear behind one address at once. Offices, schools and cafes do the same. So an IP address points at a network far more often than at a single person.

What an IP address reveals

• A rough location. Lookup services guess a country, often a region or city. The guess can be wrong by many miles, and it is rarely your street.
• Your network. Which ISP or mobile network you use, and sometimes the name of a company or university if you are on its network.
• Patterns over time. Home addresses often stay the same for days or weeks, so repeat visits from one address can be grouped together.

What it does not reveal

A website cannot turn an IP address into your name by itself. The link between an address and the person paying for the connection is held by the ISP, which does not share it with ordinary websites.

The bigger risk is combination. An IP address plus a login, an email address or other details can connect visits that would otherwise stay separate. The fingerprinting module shows how trackers combine small clues like this.

A common misconception

"Hiding my IP makes me anonymous." A VPN or relay swaps your address for one of theirs, which hides your rough location and network from sites. It does not remove cookies, logins or fingerprints, and the VPN company can now see which sites you connect to, instead of your ISP. It moves trust; it does not remove the need for it.