A simple type of proxy server filtering or monitoring. Either by matching a domain or a word in a URL, the software can block access or add to an alert log file. For example, you could add domains like "mp3s_for_free.com" to block the specific site or just add "mp3" to block any site with "mp3" in its name or path. Even http://www.mp3s-suck.com or http://www.cnn.com/news/mp3/20010821.html would be blocked by filter, though, and so you see why URL filtering is not usually a good idea for blocking sites. Rather, it is better for monitoring - where someone can first look at the page to judge if it breaks the organization's internet access rules.

URLs were just for the example and have been faked