Determining an 'active' user count of an ASP.NET site

Question

On an ASP.NET site, what are some techniques that could be used to track how many users are logged in to the site at any given point in time?

So for example, I could

Answer 1

Does the website log the user in? If it does, then you can update a "Last Visit" field in the user table every time they request a new page, and then every hour, just do a SQL query that grabs everybody who has a "Last Visit" timestamp within the last 15 minutes or so (assumed to currently be on the site).

If you're not having people log in, you could just as easily do it by IP address instead of username. With this method, though, you may run into some proxy issues (ie multiple users from the same corporate network may all come from a single IP address, so they only count as one user in your totals), but that should be minimal.