Firewall

A firewall gates root access to parts of a computer, limiting users' ability to access certain information stored on it, functionality provided by it, or modules installed on it.

Unlike other countermeasures, a firewall simply partitions modules much like walls partition parts of a building — they cannot harm or take actions against a hacker, they simply prevent access unless they are hacked. A computer can have multiple separate firewalls, but each module within a computer is typically protected by only a single firewall.