n.
Computer security system that controls the flow of data from one computer or network to another.
Firewalls are mainly intended to protect the resources of a private network from being directly accessed by a user from an external network, especially via the Internet. Users inside the private network may also be prevented from directly accessing external computers. To accomplish this, all communications are routed through a "proxy server" that determines whether a message or file will be allowed to enter or exit the private network.