A firewall is a network security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules. A firewall typically establishes a barrier between a trusted network and an untrusted network, such as the Internet.

Firewalls are categorized as a network-based or a host-based system. Network-based firewalls can be positioned anywhere within a LAN or WAN. They are either a software appliance running on general-purpose hardware, a hardware appliance running on special-purpose hardware, or a virtual appliance running on a virtual host controlled by a hypervisor.

Firewall appliances may also offer non firewall functionality, such as DHCP or VPN services. Host-based firewalls are deployed directly on the host itself to control network traffic or other computing resources.