NIST-SC-7(25) Unclassified National Security System Connections

Prohibit the direct connection of [Assignment: unclassified national security system] to an external network without the use of [Assignment: boundary protection device].

A direct connection is a dedicated physical or virtual connection between two or more systems. Organizations typically do not have complete control over external networks, including the Internet. Boundary protection devices (e.g., firewalls, gateways, and routers) mediate communications and information flows between unclassified national security systems and external networks.