Page "Ad filtering" Paragraph 16

This method exploits the fact that most operating systems store a file with IP address, domain name pairs which is consulted by most browsers before using a DNS server to look up a domain name.

By assigning the loopback address to each known ad server, the user directs traffic intended to reach each ad server to the local machine.

Running a suitable web server locally the ad content can be replaced with anything the user wishes.