<authentication mode="Forms">
      <forms loginUrl="Index.aspx"
             slidingExpiration="true"
             defaultUrl="list.aspx"
             timeout="600" name=".Auth"
             protection="All"/>
</authentication>

<system.web>

<authentication mode="Forms">
</authentication>

<authorization>
    <deny users="?"/> <!--acá denegas el acceso a los usuarios anónimos-->
</authorization>

</system.web>