Originally posted by: Goosemaster
Stateful packet inspection is a critical part of a good firewall. It basically means that you can set rules that govern the traffic of EVERY packet that passes through the firewall. Based upon these rules data is either allowed or rejected.
My Firewall for example declines any WAN to LAN requests unless they are FTP, but it is relatively simple. In a complex netwirk with various subnets, it gets complicated. just as a SWITCH has IP tables to route traffic, the firewall keeps a rules table and allows or denies requests.
A perfect example is kazaa. With a SPI Firewall, you can block kazaa or re-route it to a different port. With more capable devices QoS(quality of service) or packet shaping can be implemented allowing you to prioritize traffic etc, but that is something else