I am confused about when you have to declare an assembly or simply to import

(or Using) a namespace? 

I copied the following out of a sample application: 

<%@ Assembly name="System.Net.dll" %> 

<%@ Import Namespace="System.IO" %> 

<%@ Import Namespace="System.Data" %> 

<%@ Import Namespace="System.Net" %> 

Thanks, 

Scott