Monitor, control, and protect organizational communications (i.e., information transmitted or received by organizational information systems) at the external boundaries and key internal boundaries of the information systems.

The goal of this control is to protect your systems from network-based attacks using firewalls. To do this firewalls separate your company's networks from attackers. Firewalls also control the traffic allowed across your network. These two features prevent and limit the effectiveness of network-based attacks.

Deploy a firewall between your company network and the internet. Configure your firewall to only allow authorized traffic to enter and exit your network. Use a web proxy to block access to malicious websites. Achieve this by blocking unnecessary services and ports. If you have several internal networks, deploy a firewall between them. Only allow necessary traffic between your networks. Create a network diagram that displays your information system boundary.