What approach should be for open voting system

I am working on a website that has car profiles. There is also rating involved for each profile. Now is there is any good approach to have authentic voting system without making user register?

for example.

there is a profile named 350Z Nissan. Now users would be allowed to rate it. How can I avoid Fake rating?

I thought that I should save IP address along with rating but what if there is single computer at home and many people use it?

What would you do ?

Solution

Those are good steps to start:

One vote per 3/4/6/12/24 (your choice) hours per one ip for non registered users
Captcha for non registered users (prevents bots)
Limit number of registrations per day from one ip
Log every vote

You probably can't do anything more, you may also use cookies and create pseudousers (it'll filter out "lame users") but it's easy to work around.