A firewall is a combination of hardware and software used to implement a security policy governing the network traffic between two or more networks, some of which may be under your administrative control (e.g., your organizations networks) and some of which may be out of your control (e.g., the Internet). A network firewall commonly serves as a primary line of defense against external threats to your organization's computer systems, networks, and critical information. Firewalls can also be used to partition your organizations internal networks, reducing your risk from insider attacks.

Firewall technologies have entered into the mainstream. Power indicates that 91 percent of the organizations surveyed already deploy firewalls. Articles and other references covering evaluation, selection, and configuration of firewall technologies are now common in the popular press (see References at the end of this section). However, there has been little published about designing, installing, deploying, operating, and maintaining firewalls. The practices in this module will address designing, installing, and deploying firewalls.

The term firewall is taken from the structural analog whose purpose is to slow the spread of fire in a building. In the computer literature, popular press, and vendor marketing materials, the term is used in many ways. Some people use it to identify a specific hardware component or software package, while others consider the entire collection of systems and software deployed between two networks to be parts of a firewall.

Throughout these practices, we will generally use the term firewall as an adjective modifying a noun (such as system, hardware, software, product) to make the reference clear. When we use the term firewall as a noun, we mean the general concept of a technological mechanism for the enforcement of a network traffic security policy. While this may seem cumbersome at times, we believe these distinctions will increase your understanding of our intent.