I am unsure what you mean. But what I would do is drop a cookie with a
(semi) unique number and store that number in a database. Unless it is
_very_ important users can't vote more than once. The problem with
logging IP's is that there could be more than one PC behind a single
IP. And this is quite often the case.
If a user comes to your site you simple retrieve the cookie with the
number and see if the number already exists in the DB, if so the
person already voted, if not he did not.