Few months ago, I set up my linux box as a development web server and also as network router. I have chosen Debian GNU/Linux. Now, at last, I need to set up firewall. What I need to do is configure IPtables so, at first, my network is secure (Now, it is set up only to forward all to all, based on very simple howto, I've googled out, so my router is "functional".). At second I need to set up masquerade, so no one outside my LAN can not determine number of computers inside (I've heard, that this could be achieved by modifing packet IDs, so it is impossible to determine this by finding packet seqences.).
I am primarily programmer, not linux administrator and my linux knowledge is very limited. Could anyone help me, please, to set up real firewall/router?

I normally use NetBSD to make my firewalls, so I couldn't provide any help with IPTABLES, but I can tell you this: I read an article about the same issue you're describing. I wouldn't worry about it-- the article I read (from IBM, if I'm not mistaken) was theoretical, and there are no practical applications of the technique described currently available. I would focus more on ensuring no one could access your router inappropriately, and that undesired traffic from your LAN was being blocked by your router.