Is there a way for a web server to identify a particular computer? Maybe by seeing it's MAC address? (not sure if you can access that without java.
closed as off topic by Zoredache, SvW, Kenny Rasschaert, MadHatter, mdpc Apr 3 at 20:16
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Yes. Ish. There really isn't any way to do what you want under your constraints of being a webserver, however, having said that, there are a couple of techniques that try and are somewhat effective.
Other than cookies (and Flash cookies), the most popular method of trying to identify unique clients is a technique known as browser fingerprinting, which does require client-side plug-ins to work effectively. However, this is not a definitive test by any means, as it fingerprints the browser, not the computer... and people can and do use different browser, update their browser and change their plugins from time to time, not to mention that not all browser configurations are uncommon, so it's far from perfect. But it's really all there is, along with the use of files stored on a client's computer (cookies).