A Web filter, which is commonly referred to as "content control software", is a piece of software designed to restrict what websites a user can visit on his or her computer. These filters can work using either an allowlist or a denylist: The former allows access only to sites specifically chosen by whoever set up the filter, and the latter restricts access to undesirable sites as determined by the standards installed in the filter. These programs look at the URL of the desired site and search through the site's content for restricted keywords, and then decide whether to block or allow the connection. Filters are often installed either as a browser extension, as a standalone program on the computer, or as part of an overall security solution. However, they can also be installed on the network side, either by an ISP or a business, to restrict the Web access of multiple users at once. Some search engines also feature rudimentary filters to remove undesirable pages from search results.
Web-filtering software has two main customer bases: Parents who wish to prevent their children from accessing content they consider undesirable or inappropriate, and businesses that want to prevent employees from accessing websites that don't pertain to their jobs. Web filters are also commonly used as prevention tool for malware, as the filters will block access to sites that commonly host malware, such as those related to pornography or gambling. The most advanced filters can even block information that's sent out over the Internet, to ensure that sensitive data isn't released.
There are ways around web-filtering software, such as using a Web-based proxy, using foreign-language websites or creating a VPN to a personal proxy server. Because of these loopholes, network admins or concerned parents have to ensure that their chosen filter can do more than just block or allow certain websites.