How do Reddit and similar sites keep users from voting on links and comments more than once?

I was curious and downloaded the code but I can't seem to find what I'm looking for. I thought I might find a database table with user ids mapped to link ids or comment ids. But I can't seem to find anything 开发者_运维技巧like this.

This ticket points to some of the code involved: http://code.reddit.com/ticket/520