FAR 52.204-21 and CMMC 2.0 Level 1 Control SC.L1-B.1.XI require that public-facing components be separated from your internal networks — either physically or logically — to reduce attack surface and limit lateral movement; this post provides a practical, audit-ready checklist for small businesses to design, implement, and evidence compliant subnetwork separation.
Why separation matters (high level)
Separating public-facing systems (web servers, APIs, mail relays, VPN gateways) from internal assets prevents external attackers who compromise a public service from directly accessing sensitive internal resources. For small businesses under FAR/CMMC, segmentation is a compensating control that directly supports confidentiality and integrity of contractor information and demonstrates reasonable safeguarding. Beyond compliance, good segmentation reduces blast radius, simplifies monitoring, and makes incident response faster and more predictable.
Practical checklist — design and scoping
1) Identify public-facing components and scope: list every publicly routable hostname/IP, hosted service, and reverse-proxy endpoint. 2) Decide separation type: physical (separate switches/VLANs on discrete hardware) or logical (VLANs/VRFs, cloud VPC subnets). Small businesses typically use logical separation for cost reasons; document why this choice satisfies the framework. 3) Create a network diagram (annotated) that shows IP ranges, subnets, routing, firewall placement, and administrative jump hosts — this diagram is primary evidence for auditors.
Practical checklist — architecture and technical details
4) DMZ / public subnet: place all public-facing hosts in a DMZ subnet (e.g., 10.0.3.0/24) with strict ingress rules allowing only required ports (typically TCP/80, TCP/443). 5) Internal subnets: put application servers, databases, and file shares in private subnets (e.g., 10.0.1.0/24) with no direct inbound from the internet. 6) Management subnet: reserve a small management subnet (e.g., 10.0.2.0/28) and restrict management ports (SSH/RDP) to authenticated VPN or bastion host IPs only, never expose them broadly. 7) Example firewall rule set: inbound to DMZ: allow TCP/80,443 to DMZ hosts; inbound to internal: deny from internet; DMZ->internal: allow only specific ports (e.g., TCP/8443) from specific DMZ host IPs; all other flows: deny. Keep explicit deny rules and least privilege.
Cloud-specific implementations and a small-business example
8) Cloud implementations (AWS/Azure/GCP) map to these principles: create a VPC with a public subnet for the reverse proxy/load balancer and a private subnet for app/data; use Security Groups/NSGs to enforce the port-level rules and route tables to restrict direct Internet routes. Example: a small marketing agency hosting a client portal uses AWS — public ALB in public subnet forwards only HTTPS to web servers in a private autoscaling group, web servers fetch updates via a NAT gateway, admin access uses AWS SSM Session Manager with a centralized logging bucket. This meets logical separation and provides evidence (SG IDs, route tables, flow logs) for auditors.
Monitoring, hardening, and evidence for compliance
9) Logging and monitoring: enable VPC Flow Logs, firewall logs, web server access logs, and centralize to a SIEM or cloud logging (CloudWatch, Azure Monitor, Stackdriver). Retain logs per contract policy (document retention period). 10) Hardening: place a WAF in front of public endpoints, enforce TLS 1.2+/strong cipher suites, keep OS and app patches current, and use automated vulnerability scans at least monthly. 11) Evidence package: export firewall rule sets, security group configs, network diagrams, change control records, vulnerability scan reports, and a short narrative mapping these artifacts to FAR/CMMC control SC.L1-B.1.XI.
Risks of non-implementation and remediation priorities
Failing to separate public-facing components increases risk of credential theft, lateral movement to internal assets, data exfiltration, and contract penalties under FAR/CMMC. For small businesses, the fastest remediation path is: (A) isolate all public hosts into a dedicated subnet or security group, (B) block inbound traffic to internal subnets, (C) restrict management access to VPN/bastion and rotate credentials, and (D) document everything — auditors prioritize demonstrable boundaries and compensating controls.
Compliance tips and best practices
Make segmentation part of change control: require network diagram updates and firewall rule reviews for any new public-facing service. Use automation (Terraform/ARM/CloudFormation) to make your network reproducible and to export configs for audits. Use host-based controls (firewall, SELinux) as layered defenses but avoid relying on them alone. For proof of logical separation, collect timestampsed exports of ACLs/security groups, flow logs showing denied traffic, and screenshots of console rule entries — auditors accept exported JSON/YAML rules as machine-readable evidence.
In summary, meeting FAR 52.204-21 and CMMC SC.L1-B.1.XI for public-facing components is a practical exercise of inventory, design, enforcement, monitoring, and evidence collection: isolate public services in a DMZ or dedicated subnet, apply least-privilege firewall rules, restrict management channels, enable comprehensive logging, and maintain clear artifacts for auditors — a little upfront design and automation will keep your small business secure and compliant without large capital expense.
