I've been helping someone set up a network and the security model that seems to be in place is this:

RedHat 7.2 , 2.4, iptables

There are multiple server machines (news, mail, web etc) sitting behind a machine with a packet filtering firewall which denies all incoming except on those ports that have been explicitly declared as okay. Legit traffic is then forwarded (based on the port number) to the appropriate server.

that's the basic setup. (If I've left out any pertinent info lemme know)

So then the question is this: how strong of a setup is this? This is the first time I've really had a "real" network with a "real" connection to the net (faster then 56K) to administer and so I'm fairly ignorant about some of the security and design issues of firewalls.

Any help/comments/flames/whatever are appreciated.