The Domain Name System (DNS) is a structural element of the internet that is used to convert website address names into their numerical equivalents so that they can be found on the network and their contents displayed to the user.
DNS filtering refers to a technique that is used within the DNS to block known malicious websites and other inappropriate or harmful content. It is a part of an overall access control strategy and enables administrators of computer system to protect their data by having control over what sites can be accessed by users on the networks they manage.
Filtering occurs by comparing user-requested website addresses against a block list of known harmful addresses. Block lists are maintained by the security community, by system administrators and by individuals. Whenever a restricted address is requested access is blocked and a message to this effect is returned to the user.