There are two major threats that can stop your VPN from working:
1. WebRTC leaks: Send your real IP address and location to websites, even though you’re using a VPN.
2. IPv6 leaks: Allow some websites to see a different version of your original IP. This exposes your location.
Most VPNs block WebRTC and IPv6, but their leak protection isn’t always effective. The only way to know that you’re safe is to run a WebRTC/IPv6 leak test.
Explanation of the problem