Isolating publicly accessible systems into distinct subnetworks (DMZs / public subnets) is a core technical step to comply with FAR 52.204-21 and CMMC 2.0 Level 1 Control SC.L1-B.1.XI—this post walks through concrete, small-business friendly design and implementation steps, real-world examples, compliance evidence you should collect, and the risks of not segmenting public systems.
Why network segmentation matters for this Compliance Framework control
FAR 52.204-21 requires contractors to implement basic safeguarding for federal contract information, and CMMC Level 1 expects controls such as SC.L1-B.1.XI to ensure publicly accessible systems are separated from internal resources. Network segmentation reduces lateral movement, limits exposure of internal systems (including CUI or other restricted data), and makes it easier to demonstrate that public-facing assets cannot directly reach internal networks without explicit, auditable rules.
Designing subnetworks and zones: practical architecture
Start with a simple zone model: Perimeter (Internet), Public/DMZ (web servers, public APIs, VPN portal front ends), Internal/Workload (internal application servers, databases), Management (switches, SIEM, admin jump hosts), and Guest Wi‑Fi (completely isolated). For a small business using private addressing, a minimal IP plan might be: Public/DMZ = 10.0.10.0/24, Internal = 10.0.20.0/24, Management = 10.0.99.0/24, Guest = 10.0.200.0/24. Use VLAN tagging to keep these subnets physically consistent across switches and enforce routing only at the firewall/router layer.
Implementation steps and technical specifics
Concrete step-by-step: 1) Inventory all public-facing services (websites, APIs, VPN endpoints, IoT portals). 2) Create a DMZ/public subnet and move those services there. 3) Configure perimeter firewall/NAT so inbound traffic is only allowed to specific host IPs and ports (e.g., TCP 80/443 to 10.0.10.10). 4) Strictly deny DMZ -> Internal by default and add allow rules only when required and documented (for example, allow 10.0.10.10 -> 10.0.20.15 TCP 5432 if the web app requires DB access). 5) Place admin interfaces on the Management VLAN and never expose them to the DMZ or guest networks. Example firewall rule (expressed conceptually): "ALLOW WAN -> DMZ: tcp/80,443 to 10.0.10.10; DENY DMZ -> Internal: ANY; ALLOW DMZ -> Internal: tcp/5432 from 10.0.10.10 to 10.0.20.15 only." If using iptables on a perimeter host, the equivalent would be forwarding rules; if using a managed appliance (pfSense, Ubiquiti, FortiGate, Cisco, Palo Alto), implement policy objects and log every rule.
Cloud and small-business scenarios with examples
Small-business examples: (A) Public website hosted on-premises: Put the web server on the DMZ VLAN 10.0.10.0/24 and use NAT with your edge firewall to forward 80/443 to 10.0.10.10; deny the DMZ direct access to internal file shares. (B) Cloud-hosted public API: Create a public subnet for your load balancer and private subnets for app servers; use security groups/NACLs to permit only the load balancer to reach app servers on required ports. (C) Guest Wi‑Fi or IoT: Put all guest/IoT on a segregated VLAN with no route to internal systems and only outbound Internet access via the firewall. (D) Remote admin: Use a bastion host in the Management subnet with MFA (or an approved VPN with MFA) and require jump-host usage rather than exposing management ports directly to the Internet.
Monitoring, logging, testing and evidence for auditors
Collect evidence to show compliance: a current network diagram that labels subnets and VLAN IDs, firewall rule exports (timestamped), NAT/port-forwarding entries, router ACLs, change-control records showing the DMZ creation, and screenshots of cloud security group rules if applicable. Enable and retain logs: firewall connection logs, NAT translations, and system logs from public hosts. For cloud, enable VPC Flow Logs and store them centrally. Regularly validate segmentation with internal scans and penetration tests—simple Nmap scans from an external vantage point and from DMZ hosts should confirm that internal hosts are not directly reachable. Recommended retention: keep firewall logs for the period your compliance policy requires; if no policy exists, retain at least 90 days and archive longer if possible.
Compliance tips and best practices
Practical tips: (1) Apply "deny by default" between zones—only open specific ports for specific source/destination pairs. (2) Use service accounts and restrict service-to-service permissions rather than opening wide network access. (3) Keep a canonical asset inventory tied to IP addresses so auditors can see what’s public. (4) Automate rule deployment where possible (IaC for cloud security groups or configuration templates for appliances) and keep versioned change logs. (5) Harden public hosts: minimal services, latest patches, WAF in front of web apps, and TLS everywhere. (6) For VPN or remote admin, require MFA and place jump hosts in the Management VLAN rather than DMZ. (7) Perform periodic segmentation testing and update diagrams after any network changes.
Risks of not implementing subnetworks for public systems
Failing to segment public systems increases exposure: an exploited public web server can become a pivot point into internal systems, exposing CUI or sensitive business data and creating a breach that may result in contract penalties under FAR and failing CMMC assessments. Other risks include malware spread to internal workstations, unauthorized access to administrative interfaces, and lack of demonstrable controls during audit—making remediation more costly and time-consuming.
Summary: For Compliance Framework requirements such as FAR 52.204-21 and CMMC 2.0 SC.L1-B.1.XI, implementing clear subnetworks for public systems is both a technical and audit-driven necessity—plan simple zones, enforce deny-by-default firewall policies, document and log everything, test segmentation regularly, and use pragmatic small-business solutions (VLANs, DMZ, firewall rules, WAF, bastion hosts, and cloud security groups) to minimize risk and provide evidence for assessments.
