Mobile Networking · Mobile Data Under the Hood · Lesson 2 of 4

Hotspots & Tethering

Share your phone's mobile data with other devices, and learn what that reveals.

Three Ways to Share

Sharing your phone's mobile data with another device is called tethering. iPhones call the feature Personal Hotspot. Android calls it hotspot and tethering. Both offer three ways to connect.

• Wi-Fi hotspot. The phone acts like a small Wi-Fi router and broadcasts a network name. Laptops and tablets join it with a password.
• USB tethering. You plug the other device in with a cable. Nothing is broadcast, and the phone can charge at the same time.
• Bluetooth tethering. The devices pair over Bluetooth. It's usually slower, but it works when Wi-Fi isn't an option.

Whichever method you pick, the phone does the same job as your home router. It hands each connected device a private address, then runs NAT to share its own single cellular connection.

Here's an everyday example. You're on a train with a laptop and no Wi-Fi, so you turn on your phone's hotspot and get to work.

A Wi-Fi hotspot's name is visible to anyone nearby. Use a strong password, and turn the hotspot off when you're done.

What the Carrier and Websites See

Hotspot use depends on your plan. Some plans include a set amount of hotspot data, some slow it down after an allowance, some charge extra, and some don't allow it at all. Google's own Android help tells you to check with your carrier first, because rules vary by carrier and plan.

Carriers can often tell hotspot traffic from phone traffic. On some networks the phone sends tethered traffic through a separate APN, which makes it easy to count and limit.

To the rest of the internet, a hotspot is invisible. Your laptop's traffic leaves through the phone's cellular connection, so websites see the phone's IP address, usually a shared CGNAT address. They have no direct view of the laptop's private address behind it.

This matters in investigations. If a laptop on a hotspot does something that ends up in a server log, the carrier's records point to the phone's subscriber, who may not be the person using the laptop. Investigators then need other evidence, such as the devices themselves, to work out who did what.