High Quality Content by WIKIPEDIA articles! A firewall is a system or group of systems (router, proxy, or gateway) that implements a set of security rules to enforce access control between two networks to protect "inside" network from "outside" network. It may be a hardware device or a software program running on a secure host computer. In either case, it must have at least two network interfaces, one for the network it is intended to protect, and one for the network it is exposed to. A firewall sits at the junction point or gateway between the two networks, usually a private network and a public network such as the Internet.