To specify configuration which only applies to a specific file, use the
<location>
element:
location Element (ASP.NET Settings Schema)[
^]
<system.web>
<authentication mode="Forms">
...
</authentication>
</system.web>
<location path="admin.aspx">
<system.web>
<authorization>
<allow users="Admin"/>
<deny users="?"/>
</authorization>
</system.web>
</location>