The Domain Name System (DNS) converts website domain names (site.com) to IP addresses (126.96.36.199) so that they can be found on the network. Whenever a request for data from a website is received, it passes through the DNS so that the website name resolves to its IP address.
DNS filtering refers to the process of blocking certain sites from being resolved when they are received at the DNS. These could be malicious sites or sites that contain illicit content. The way that the DNS blocks sites is by checking whether they are present on a blocklist. The blocklist contains the name of sites that must not be resolved (that is, are blocked) by the DNS. The DNS can block websites either by domain name of an IP address since the blacklist contains a list of all known harmful domain names and IP addresses.