Mobile Networking · Network Privacy · Lesson 1 of 4

VPNs: What They Hide

See what a VPN hides from the local network and who can see your traffic instead.

The Café View, With and Without a VPN

In the last course you saw what a café Wi-Fi owner can see when your phone browses over HTTPS. Page contents stay encrypted. The names of the sites you visit still leak through DNS lookups and the TLS server name.

A VPN, or virtual private network, changes that picture. Your phone wraps all of its traffic in an encrypted tunnel to a server run by the VPN provider. The café network only carries the tunnel. With a properly working VPN, the owner can see that your phone is talking to one VPN server, plus the timing and amount of data. They can't see where the traffic goes after that.

The VPN server unwraps your traffic and sends it on to the real destinations. To those websites, your requests seem to come from the VPN server's IP address instead of the café's.

On the simulated phone, browse a few sites with the VPN toggle off and look at the destinations in the café owner's view. Then switch the VPN on, browse again, and count how many different servers the café owner sees.

Who Sees Your Traffic Now

A VPN moves the view of your traffic to a new place. The café or your mobile carrier sees less, and the VPN provider now sits where they used to sit. The provider's server can see which sites you connect to, when, and how much data you send. HTTPS still protects page contents on the trip from the VPN server to the website.

So picking a VPN is a decision about trust. You're betting the provider is more trustworthy than the network you're on. Many providers promise to keep no logs. Some have had those promises checked by independent audits, and others have turned out to keep records anyway. Free VPN apps need to make money somehow, and researchers have found some of them collecting and sharing user data.

A VPN also leaves a lot alone. If you sign in to a shopping site, the shop knows it's you. Cookies, advertising IDs and trackers inside apps keep working as before. A VPN is a good tool for untrusted Wi-Fi and for hiding your IP address from websites. It won't make you anonymous.