The Domain Name System (DNS) is a like an address book for websites; it translates the names of websites that we are familiar with into numerical form so that they can be found on the internet and their contents displayed.
DNS filtering is a process where, whenever a request is received to resolve a website name into its address, it is checked against a blocklist which, like its name suggests, blocks all sites that are contained in it. This is used to block known malicious websites, such as those used for fraud or identity theft. The DNS filter uses blocklists of known sites drawn from previous crawls of the internet, sites that have been reported and sites that are judged malicious in real time. When a site is blocked the request will be redirected to a page that explains to the user why the site cannot be accessed.