how uniquely identify a website visitor, like thetoptens.com [on hold]

I test this site for voting www.thetoptens.com

My Questions is how this site work to uniquely identify a website visitor for voting

, while

1. don't need to register for voting
2. same IP but different device can vote (for example your laptop and smartphone have same IP on one router but each one can vote one time )
3. by change cookies browser can't vote more than one time.
4. by change mac you can't vote more than one time.

Attention: when you remove cookies can voting more than one time on same post, but your vote don't effect and don't up percent of voting. (you can test it by create your own list)

I even testing vmware for another windows but can't voting more than one time on same list that I created. This site saprate uniqe visitor