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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 (private or company) network and an untrusted network, such as the Internet.

Firewalls are categorised as a network-based or a host-based system. Network-based firewalls can be positioned anywhere within a Local Area Network (LAN) or Wide Area Network (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. Host-based firewalls are deployed directly on the host itself to control network traffic or other computing resources. Functions a firewall can hold include packet filtering, connection tracking (circuit level gateway), application layer filtering (protocol abuse, deep packet inspection) and socket filtering.

Related Keywords: barrier, router, trusted network, next-generation firewalls, intrusion prevention system, identity management