<authentication 
   mode="[Windows|Forms|Passport|None]"> 
   <forms>...</forms>
   <passport/>
</authentication>

http://msdn.microsoft.com/it-it/library/532aee0e(v=vs.85).aspx

Il Tag AUTHORIZATION
Configura l'autorizzazione per un'applicazione Web, con il controllo dell'accesso client alle risorse URL. 

<authorization> 
   <allow .../>
   <deny .../>
</authorization>

predefinita:
<authorization>
   <allow users="*" />
</authorization>

http://msdn.microsoft.com/it-it/library/8d82143t(v=vs.85).aspx