Make internal resources available:
Although the primary purpose of a firewall is to prevent unwanted network traffic from passing through it, you can also configure many firewalls to enable selective access to internal resources, such as a public web server, while still preventing other access from the Internet to your internal network. In many cases, you can accomplish this by using a DMZ, which is where the public web server would be located. (DMZs are discussed later in the section “Essentials First: Life in the DMZ.”)