Calculating Number of Users Based on Amount of Unique IPs?

pjdepasq asks: "I run a small but growing web site. Currently the site has optional registration (for the message boards), though we know we have a larger number of anonymous users. Is there an industry standard for calculating number of unique users based on the unique IP addresses over a period of time (1 week? 1 month?) We'd like to get a handle on the number of users we have. Sure, I know about dynamic IP addreses and ISPs like AOL which can dilute or confuse the numbers, but surely there's some benchmark calculation we can use."