try like this
in VDETAILS.ASPX page :
under page load:
if(session["name"]=="")
{
response.redirect ("default1.aspx");
}
or else put the address bar readonly
see this link
<a href="http://forums.asp.net/t/1207625.aspx/1">http:
<a href="http://www.codeproject.com/Questions/147018/how-to-hide-address-bar-in-asp-net-application">how to hide address bar in asp.net application</a>[<a href="http://www.codeproject.com/Questions/147018/how-to-hide-address-bar-in-asp-net-application" target="_blank" title="New Window">^</a>]