Firewall

A firewall is a security system, in hardware or software, that monitors network traffic and allows or blocks it according to a set of rules. It acts as a barrier between a trusted network, such as a company LAN, and untrusted networks such as the internet.

Firewalls can filter by IP address, port and protocol, track the state of connections, and next-generation firewalls inspect application content too. Operating systems include software firewalls for individual computers, while routers and dedicated appliances protect whole networks.