I am no electronic/computer engineer and I am assuming you aren't either, with that in mind, I know it’s possible to do because I’ve seen it beign done by poker sites. But more specifically on ip: this IP you get from the modem is not unique in the sense that, yes, it changes all the time and sometimes more than one device will have the same IP (there’s only about 4 billion ipv4 numbers and there is more than 4 billion devices using the internet in the world). But this is not truly how someone identify your internet address, because your internet is managed by a network provider who has a set of IP address based on the location and the time you are using it, so even if your ip changes a few times a day, it changes in a specific way or, at least, is recorded in a specific way and can be accessed and you can very precisely pinpoint were a user is connecting from, especially if you are connecting his device straight to your own server. For example, in Germany, people receive letters/bills and are prosecuted for downloading illegal softwares/copyrighted material. And this is not the only way you can do that: you can establish cookies that track your connection using Google permissions, for example. For the email part, it could be a phone call. I am a poker player and pokerstars employees call your registered phone number and ask about specific hands you played and stuff like that when they are suspicious of account sharing or something like that. Tibia employees could call you and make questions related to gameplay and stuff like that.