Requirement
Show: SC.L1-B.1.X – 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.
This control comes from FAR 52.204-21 / CMMC 2.0 Level 1.
Understanding the Requirement
This control requires you to clearly define where your network begins and ends (external boundary), identify important internal separations (key internal boundaries), and then consistently monitor, control, and protect traffic crossing those lines. In practice, that means using firewalls and related controls to restrict which services and ports are allowed, filtering web access to block malicious destinations, and keeping an eye on traffic and logs so you can prevent and limit the impact of network-based attacks.
Policies and Procedures Needed
Establish a Network Boundary Security Policy that defines your external boundary (e.g., internet edge), key internal boundaries (e.g., servers, finance, POS, guest Wi‑Fi), and the authorized traffic allowed between them. Create procedures for firewall rule management (request, approval, implementation, and review), web filtering and DNS filtering administration, and log monitoring with escalation steps. Include standards for network diagrams and data flows, change management for boundary changes, periodic access/rule reviews (at least quarterly), and incident response steps when unauthorized traffic or malicious sites are detected. Assign responsibility to system/network administrators and information security roles for oversight and documentation.
Technical Implementation
- Deploy and configure an edge firewall at the internet boundary; default to deny inbound traffic, and allow only business-required outbound traffic (e.g., HTTPS, required VPNs or SaaS services). Disable or remove unused services and block unnecessary ports.
- Define key internal boundaries and segment your network (e.g., separate VLANs/segments for servers, workstations, VoIP, POS/operational tech, and guest Wi‑Fi). Place internal firewalls or router ACLs between segments to allow only necessary protocols (for example, workstations to servers on HTTPS/RDP as authorized).
- Implement web protection using a secure web gateway, proxy, or DNS filtering to block malicious, phishing, and high‑risk categories (e.g., malware, gambling, adult). Enforce HTTPS inspection only where appropriate and lawful, and maintain an exception process.
- Turn on firewall and router logging for allow/deny events; forward logs to a centralized log tool where feasible. Create basic alerts for repeated outbound blocks, denied inbound attempts, or traffic to known malicious domains/IPs, and review logs at least weekly.
- Maintain a current network diagram that labels the external boundary, key internal boundaries, routing paths, and enforcement points (firewalls/ACLs/proxies). Update it after any change and review it during quarterly rule audits.
- Harden remote access: require VPN for administrative access, disable direct RDP/SSH from the internet, and restrict VPN access to necessary internal segments with multifactor authentication.
Example in a Small or Medium Business
Riverview Design, a 40‑person firm, deploys a business‑class firewall between its ISP modem and the office switch. The IT lead documents the external boundary as the internet edge and defines key internal boundaries for servers, staff workstations, and guest Wi‑Fi using separate VLANs. In the firewall, inbound traffic is fully blocked, with a single VPN service exposed for remote admins using MFA. Outbound, only HTTPS and required VoIP services are allowed from staff, while the guest network is restricted to web traffic and isolated from all internal segments. The company enables DNS filtering to block malware, phishing, adult content, and other risky categories, and sets alerts for repeated blocks or connections to known bad domains. Weekly, the IT lead checks firewall logs and addresses any anomalies; quarterly, they review and prune rules and update the network diagram. When finance needs access to the file server, the IT lead adds a rule permitting only SMB over a secure channel from the finance VLAN to the server VLAN and records the change through a simple approval workflow. These steps ensure traffic crossing the external and internal boundaries is monitored, controlled, and protected in line with policy.
Summary
By defining your external and internal boundaries, enforcing least‑privilege traffic with firewalls and segmentation, and adding web/DNS filtering and routine log review, you meet SC.L1-B.1.X’s mandate to monitor, control, and protect communications at critical network boundaries. Clear policies for boundary definitions, firewall rule management, and log monitoring—paired with practical implementations like deny‑by‑default rules, segmentation, and filtered web access—reduce attack surface and limit the impact of network threats. With current diagrams, periodic reviews, and basic alerting, SMBs can maintain a defensible network posture that aligns with FAR 52.204-21 / CMMC 2.0 Level 1 expectations.
