IP addresses are interesting and useful if you know their provenance but should not be relied upon to uniquely identify a user.
Be careful when using this value. Proxy servers and firewalls can modify the value you actually see and can completely mask the IP address of the client. This can make all users behind the firewall or proxy appear to be using the same machine.
Multiple users running Netscape Navigator via an X-Windows environment may indeed all be running the browser core on the same machine and only viewing the windows on their desktop systems.
Finally, clients connected via an ISP may have floating IP address values and may be served through proxy and firewall systems. Indeed, some ISP-based users may even submit each request during a session through a different firewall. The session state can then only be identified by means of hidden values in the URL or with a cookie.