Get top level URL Domain

Question

Hi everyone. I'm trying to modify the WebBrowser control in order to limit where users can go. In order to do this, I need to know the top level domain for all sites I go to. For example, if I go to www.wwe.com and then click on the superstars link, the new site would be www.wwe.com/superstars, but the top level domain is still www.wwe.com. I need a way to check that top level domain. Please help.