CyberSecurity FAQ - What is a firewall and how does it work?

Background: In general usage, a firewall is a fire-resistant barrier that is used to prevent the spread of fire for a prescribed period of time. Fire walls are built between or within buildings, or within an aircraft or vehicle. In the context of computer networks, a firewall is a network security system that monitors incoming and outgoing network message traffic and prevents the transmission of malicious messages based on an updatable rule set. In effect, a firewall establishes a barrier between a trusted, secure internal network and external networks (e.g., the Internet) that are assumed to be untrustworthy and non-secure. Firewalls can be implemented as software that runs on general-purpose hardware (e.g., an open source firewall on a Windows PC or Mac OS X computer) or a dedicated hardware device (appliance). How does a firewall work? In essence, firewalls function as a filter between a trusted, secure internal network and external networks (e.g., the Internet) that are assumed to be untrustworthy and non-secure. The firewall filter may be flexibly programmed to control what information packets are allowed and blocked.