In computing, 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 internal network and untrusted external network, such as the Internet

Further, Firewalls are often categorized as either network firewalls or host-based firewalls.

Network firewalls filter traffic between two or more networks and run on network hardware.

Host-based firewalls run on host computers and control network traffic in and out of those machines.

Firewall is a device that forms a barrier between a secure and an open environment when the latter environment is usually considered hostile, for example, the Internet. It acts as a system or combination of systems that enforces a boundary between more than one networks. Access controls are common form of controls encountered in the boundary subsystem by restricting the use of system resources to authorized users, limiting the actions authorized users can take with these resources and ensuring that the users obtain only authentic system resources.