All web sites use the domain name system or DNS to help in making it easy for identification and use, instead of the complicated numbers and other codes that make up their IP address on the web that they really work on.
It is often found necessary for users of the internet to block websites that they consider harmful, have inappropriate content, and those that impinge on their commercial interests. This can be done by using DNS filtering that ensures security and can also help companies to control the sites that their employees have access to.
When you try to access any site by using its domain name, this query Is sent to a DNS resolver that matches the name with its IP address. These resolvers can be configured to refuse queries that are on a blocklist and this prevent access to those sites.