Firewall is a network security system that monitors and controls the incoming and outgoing network traffic based on predetermined security rules.A firewall typically establishes a barrier between a trusted, secure internal network and another outside network, such as the Internet, that is assumed to not be secure or trusted.

Firewalls are often categorized as either network firewalls or host-based firewalls. Network firewalls are a software appliance running on general purpose hardware or hardware-based firewall computer appliances that filter traffic between two or more networks. Host-based firewalls provide a layer of software on one host that controls network traffic in and out of that single machine.

Routers that pass data between networks contain firewall components and can often perform basic routing functions as well, Firewall appliances may also offer other functionality to the internal network they protect such as acting as a DHCP or VPN server for that network.