which is the method providing the web page calling another web page?

hi,
let us say 'default.htm' (.aspx is not possible due to whatever reason) is "calling" 'start.aspx' as following:
<a href="start.aspx"></a>
now i would like to have a method in 'start.aspx' letting me know, from which page 'start.aspx' had been "called" (in my sample this would be 'default.htm'). what would the c#-code look like?
 
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