Helping people with computers... one answer at a time.
VPN's (Virtual Private Networks) connect through particular ports and have particular characteristics that are visible to your ISP. I discuss what they can and cannot see.
Can my service know if I'm using a VPN? Thanks.
In this excerpt from Answercast #20, I look at the way a VPN service works and what your ISP can see you doing.
By "service," I'm going to assume you mean your ISP – your Internet Service Provider.
The short answer is very definitely yes.
VPNs connect through particular ports and have particular characteristics.
The data that goes across a VPN connection, while it is totally encrypted, has two things about it that are important and that are visible.
1) It is clear where the VPN data, that packet of data, is going. It has to go to the VPN's service.
Your ISP is responsible for taking a packet of data from you and forwarding it on to where it's supposed to go. The only way it can do that is if it knows where the packet is supposed to go.
The only way it can know where it's supposed to go is if you tell it. Part of the protocol says, "Hey, this packet needs to go over to the VPN service's servers over there." So, clearly, the ISP knows exactly where every packet of data you are sending out is going to go; at least on its initial step.
2) Similarly, VPN services tend to use specific ports.
Port 80, for example, is what's used to display web pages. Port 25 is what's used to send email. VPN services often use several specific ports that identify them as being VPN connections. So even if this ISP doesn't recognize the destination of the VPN packet of data, it can potentially detect the fact that it's a VPN by the port numbers that are being used.
In other words, you may be using the VPN to connect to websites A, B, and C and send all sorts of interesting information to those websites; or send email; or whatever. Your ISP can see none of that.
All they can see is encrypted data that they can't decrypt. So they know
you're using a VPN, but they don't know what you're using it for.