As an added security measure against port scanning, stateful inspection firewalls close off ports until connection to the specific port is requested.

Check Point Software is credited with coining the term stateful inspection in the use of its FireWall-1 in 1993.

eSecurity Planet 
A resource for daily information on e-security targeted to IT managers. The site provides users with information from a variety of sources, including experts at security product and services firms, and the consultants who follow the security industry.

Design the Firewall System
A practice from the CERT Security Improvement Modules.

Extinguishing Firewall Hyperbole
Are new firewall technologies improvements or complications? Evaluate the pros and cons.

Great Walls of Fire
Hackers, lurking cyber punks and viruses. Enough to give any network administrator the willies. But, an efficient firewall could be just the armor your network may need to combat most of these threats.(2001)