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. Your internet router generally has a built-in firewall. Configure your firewall to only allow authorized traffic in and out of your network. Achieve this by blocking unnecessary/unused websites, services and ports. Unnecessary websites include adult, gambling, and malicious websites. If you have several internal networks, deploy a firewall between them. Only allow necessary traffic between your networks.