I have a NAT router, why do I need a firewall ?
Many people assume that if they are using NAT, they can forget about firewalls and DMZs and service networks.
What is often overlooked is that anytime you allow a session to be initiated from an external network, you open yourself up to potenital attacks. Many attacks come through the few well known ports that are often open on most networks(such as 80 for HTTP, 20/21 for FTP, 25 for sendmail, 110 for POP3, 143 for IMAP, 53 for DNS, 23 for telnet, etc.) Many exploits are based on the fact that some versions of the programs that run on these ports are insecure and have weaknesses that can be exploited. All this is beyond the scope of, and not addressed by NAT/PAT devices.
A firewall, on the other side has additional features, such as precise tracking of source/destination addresses, inspecting packet contents up through the application layer, logging of external connections to internal IPs, content filters, etc. Also, a multi-layered security approach is always advised, a potential attacker must penetrate multiple barriers between your protected LAN and the public Internet.