A firewall is a network security tool designed to monitor network traffic and allow or block network activity based on a set of predetermined rules. The firewall acts as a bridge between an internal network and an external network.

Firewalls operate on the principle that the internal network is secure and the external network is insecure. Thus, in a typical firewall setup a computer on the inside of the network (such as your desktop computer at home, behind your combination Wi-Fi router/firewall) is able to freely connect out to the external network (the internet) but computers from the external network (such as those belonging to a hacker probing your network for vulnerabilities) are not allowed in.

Most modern operating system have built-in software firewalls designed to protect the individual computer the operating system is installed on and most home routers (both wired and wireless) come with firewalls installed and active.