How do Internet servers know I'm using a VPN?
 

I'm currently in my annual "Prairie Winter Avoidance Mode" in Saigon. I often use a VPN, mainly for secure banking transactions and occasionally to access web sites that are blocked here - the BBC for example.

I have frequently noticed that if I have my VPN activated, some websites cannot be accessed, until I turn off the VPN.

Is it just simply that the websites in question maintain a blacklist of IP addresses that are associated with VPN servers or is there something else going on?

PPRuNe's server definitely maintains a blacklist but at least the server tells me. None of the other sites do this - they just don't respond.

I think your suggestion that there is a list of IP addresses associated with VPN servers is the most likely reason for
access denial by certain sites.

That's my understanding as well. Another possible way would be limit the number of concurrent connections from a single IP address but I don't know if any do that.

Not that practical as some ISPs use Carrier Grade NAT (CGNAT).

This puts multiple independent users on the same IP address.

I believe all the UK mobile operators use CGNAT.