Many web sites suffer from others directly linking to their image, video and other content. This practice is called often called leeching, hot-linking, or inline-linking and causes wasted bandwidth and increased server load to the victim web site.
Reference:- http://mvolo.com/blogs/serverside/archive/2006/11/10/Stopping-hot_2D00_linking-with-IIS-and-ASP.NET.aspx
Download LeechGuard