Another technique that filtering companies use in order to deal with a structural feature of the Internet is blocking the root level URLs of so-called “loophole” Web sites.  These are Web sites that provide access to a particular Web page, but display in the user’s browser a URL that is different from the URL with which the particular page is usually associated.  Because of this feature, they provide a “loophole” that can be used to get around filtering software, i.e., they display a URL that is different from the one that appears on the filtering company’s control list.  “Loophole” Web sites include caches of Web pages that have been removed from their original location, “anonymizer” sites, and translation sites.  Caches are archived copies that some search engines, such as Google, keep of the Web pages they index.  The cached copy stored by Google will have a URL that is different from the original URL.  Because Web sites often change rapidly, caches are the only way to access pages that have been taken down, revised, or have changed their URLs for some reason.  For example, a magazine might place its current stories under a given URL, and replace them monthly with new stories.  If a user wanted to find an article published six months ago, he or she would be unable to access it if not for Google’s cached version.

Some sites on the Web serve as a proxy or intermediary between a user and another Web page.  When using a proxy server, a user does not access the page from its original URL, but rather from the URL of the proxy server.  One type of proxy service is an “anonymizer.”  Users may access Web sites indirectly via an anonymizer when they do not want the Web site they are visiting to be able to determine the Ip address from which they are accessing the site, or to leave “cookies” on their browser.  Some proxy servers can be used to attempt to translate Web page content from one language to another.  Rather than directly accessing the original Web page in its original language, users can instead indirectly access the page via a proxy server offering translation features.  As noted above, filtering companies often block loophole sites, such as caches, anonymizers, and translation sites.  The practice of blocking loophole sites necessarily results in a significant amount of overblocking, because the vast majority of the pages that are cached, for example, do not contain content that would match a filtering company’s category definitions.  Filters that do not block these loophole sites, however, may enable users to access any URL on the Web via the loophole site, thus resulting in substantial underblocking. 3.  The Process for “Re-Reviewing” Web Pages After Their Initial Categorization Most filtering software companies do not engage in subsequent reviews of categorized sites or pages on a scheduled basis.  Priority is placed on reviewing and categorizing new sites and pages, rather than on re-reviewing already