Most of websites put some restrictions on how to use their services; the following paragraphs are taken from “Terms of Service” of such website:
Only one account per computer is allowed to view ads. If more than one account in a single computer view ads, all of those accounts will be permanently suspended.
User can only use a maximum of 3 distinct computers in a 10 days period to view ads. Any attempt to use more than that in that defined period will cause an account suspension.
No problem with such rules, they must be respected. But I want to know how can servers recognize a specific computer! Is there a computer fingerprint that would be transferred through HTTP connection and can identify each computer?