How Websites and ISPs Know You Are Using a VPN?

A VPN (Virtual Private Network) is a tool that helps protect your privacy on the internet. It creates an encrypted connection between your device and a remote server. When you use a VPN, your internet traffic goes through this server, and your real IP address is hidden.
VPNs are often used to stay safe on public Wi-Fi, protect privacy, avoid internet censorship, or access content that is blocked in some countries. But many websites and internet providers can still know that you are using a VPN. The question is: how?

1. IP Address Checks and Blacklists:
Every time you visit a website, you send an IP address. This is true even when you use a VPN.
Many companies keep lists of IP addresses that belong to VPN services, cloud servers, or proxy networks. If your IP address is on one of these lists, the website can guess that you are using a VPN.
Streaming services like Netflix often use this method. If they see that your IP belongs to a data center instead of a normal home internet provider, they may block the video or show only limited content.
Banks also do this for security reasons. If you usually log in from one country and suddenly your IP appears in another country, the bank may think something is wrong and block the login.

2. Location and Behavior Differences:
Websites do not only look at your IP address. They also check if your online behavior looks normal.
For example:

• Your profile says you live in the USA
• Your payment card is from the USA
• But your IP address is from Europe

This difference can make the website suspicious. Advertising companies also want to know your real location to show the right ads and to stop fake clicks or bots.
When a website thinks your connection is risky, it may show CAPTCHAs, limit features, or block access.

3. DNS, WebRTC, and Small Leaks:
Sometimes, VPN users are detected because of technical mistakes or leaks.
DNS leaks happen when your device uses your internet provider’s DNS instead of the VPN’s DNS. A website can notice that the DNS request comes from one country, but your IP is from another.
WebRTC leaks can happen in web browsers. WebRTC can reveal your real IP address through special network requests, even when a VPN is on.
Websites can also check your system time zone using JavaScript. If your IP is in one country but your time zone is in another, this can be another sign that you are using a VPN.

4. ISP Detection and Deep Packet Inspection:
Your internet service provider (ISP) cannot see the content of your VPN traffic because it is encrypted. But it can still see patterns.
When you use a VPN, most of your traffic goes to one server and is always encrypted. This looks very different from normal internet use, where traffic goes to many different websites.
In some countries, ISPs use a technology called Deep Packet Inspection (DPI). DPI can detect the special patterns of VPN protocols like OpenVPN, WireGuard, or IPsec. Even if the data is encrypted, the protocol itself can be recognized.
Some workplaces, schools, and public Wi-Fi networks also block VPN connections to stop users from bypassing rules.

5. Why It Is Hard to Hide VPN Use:
VPN providers try to hide VPN traffic using special tools like traffic obfuscation. These tools make VPN data look like normal web traffic.
However, this is a constant “cat and mouse” game. When VPNs improve, websites and ISPs also improve their detection methods.
Because of this, it is often easy to hide what you do online, but much harder to hide that you are using a VPN.

Seyed Hamed Vahedi     Wed, 4 February, 2026