But Felgall's point needs to be considered as well: Many people use large-scale ISPs or are on machines behind a company firewall. In such cases, you can have hundreds or even thousands of users (e.g., Microsoft has only a handful of outward facing IP addresses for all its thousands of emploeyees) with the same IP address. So if one person using that ISP or from that company votes, nobody else will be able to.
Now, maybe you don't care about these people, and that's really fine. Just be aware that the problem exists.
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.