What does a firewall do? A firewall is a program or device that acts as a barrier to keep destructive elements out of a network or specific computer. Firewalls are configured (in hardware, software, or both) with specific criteria to block or prevent unauthorized access to a network. They work as filters for your network traffic by blocking incoming packets of information that are seen as unsafe. In large corporations, if a firewall is not in place, thousands of computers could be vulnerable to malicious attacks. Firewalls should be placed at every connection to the internet and are also used to control outgoing web traffic as well in large organizations.