I have a site that allows webmasters to create a poll for their site. One of the features of my polls is preventing users from voting twice. I do this by recording the IP addresses. But I'm receiving many requests from users behind a firewall to prevent users from voting twice by using cookies instead of checking the IP address. Users behind a firewall have the same IP address.

I know my CGI, but I have no idea how to do this with cookies. Can someone tell me where I can get more information about this subject?