try following statement
Uri myuri = new Uri(System.Web.HttpContext.Current.Request.Url.AbsoluteUri);
string pathQuery = myuri.PathAndQuery;
string hostName = myuri.ToString().Replace(pathQuery , "");
Example1
Input : http://localhost:4366/Default.aspx?id=notlogin
Ouput : http://localhost:4366
Example2
Input : http://support.domain.com/default.aspx?id=12345
Output: support.domain.com