When I'm using the internet from my ISP 'directly' (I mean with no VPN or proxy), I can surf the web freely and almost never encounter a captcha but as soon as connecting to a VPN service, captchas are all over the place.

I know that this is a side effect of using VPNs because all the traffic is originating from the same shared IP which for services like Google search engine is very much like a spam or bot traffic.

But my question is why this 'captcha abundance' is not occurring when using the internet traffic of ISP directly? If my ISP has say for example 10,000 users, they are all behind NAT and are connecting to the internet via same shared IP, then why their traffic is not considered 'spam like' like VPN scenario?

  • Most ISP customers are NOT behind NAT, although many do have unguaranteed address assignments that may change now and then, and some newer ISPs (like cellphone networks) without enough IPv4 addresses do need to use 'Carrier Grade NAT' (CGNAT or CGN, see wikipedia). Jun 14 '18 at 6:59
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    Then how many IPs should an ISP have for 10,000 users so that their individual traffic looks unique?!
    – wiki
    Jun 14 '18 at 7:41
  • ISPs have well-defined Autonomous Systems. Most VPN provides do not.
    – Daniel B
    Jun 14 '18 at 8:29
  • My guess would be that most bots are using VPNs to stay hidden from the public Internet. Some AI must have picked it up and learned to distrust connections behind a VPN
    – marijnr
    Jun 14 '18 at 8:55

The ISP will have a range of addresses, and the carrier grade NAT (network address translation) ISPs are using will map your requests to one of these many addresses.

On the other hand, VPNs are often deployed on infrastructure where they only get a single public IP addresses, which then is shared among all users of the VPN.

And yes, today basically (nearly) all ISP customers are behind NAT, unless they pay their ISP extra for a public static address. Which typically only applies to businesses, not to private persons.

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