ASP Authentication

Hi List,
 
I am wondering what was the most secure way to allow users to access pages 
after authentication, i.e.: user authenticates in toto.asp, and after that, 
access is granted to tata_1.asp, tata_2.asp, ..., tata_n.asp. The trouble is 
obviously to ask the user once for his login / password (just in tot.asp), and 
to allow him to get to the other pages without asking each time his credentials.
 
Googling around, I saw a couple of ways to meet my needs, but all seem to be 
weak:
-  I can set a hidden field where I can say "yes, he is authenticated" or "no, 
he is not", but anyone a little bit skilled can create a fake request having 
this set up by hand (with a proxy ! ),
-  I can check a session number or smth like that on each page...but this does 
not seem very reliable,
-  I can check IP adress...but when you use AOL for instance, IP adresses can 
change !
 
So none of the ways I found seem to be the best... 
 
Cheers list, for any reply / clue !

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