Network firewalls prevent unknown programs and Internet processes from accessing the system . However, they are not antivirus systems as such and thus make no attempt to identify or remove anything. They may protect against infection from outside the protected computer or LAN, and limit the activity of any malicious software which is present by blocking incoming or outgoing requests on certain TCP/IP ports. A firewall is designed to deal with broader system threats that come from network connections into the system and is not an alternative to a virus protection system.

Single firewall

A single firewall with at least 3 network interfaces can be used to create a network architecture containing a DMZ. The external network is formed from the ISP to the firewall on the first network interface, the internal network is formed from the second network interface, and the DMZ is formed from the third network interface. The firewall becomes a single point of failure for the network and must be able to handle all of the traffic going to the DMZ as well as the internal network. The zones are usually marked with colours -for example, purple for LAN, green for DMZ, red for Internet (with often another colour used for wireless zones).

Dual firewalls

A more secure approach is to use two firewalls to create a DMZ. The first firewall (also called the "front-end" firewall) must be configured to allow traffic destined to the DMZ only. The second firewall (also called "back-end" firewall) allows only traffic from the DMZ to the internal network. The first firewall handles a much larger amount of traffic than the second firewall.

 The practice of using different firewalls from different vendors is sometimes described as either "defense in depth" or (from an opposing viewpoint) "security through obscurity".