This post explains how to design and configure cloud VPC/VNet subnetworking for public-facing services in a way that satisfies the intent of FAR 52.204-21 and CMMC 2.0 Level 1 control SC.L1-B.1.XI — minimizing exposure of systems and data while enabling necessary internet access for public workloads.
\n\nFAR 52.204-21 and CMMC Level 1 require basic safeguarding of information and cyber hygiene; a core implementation pattern is to avoid placing sensitive data and internal services on the same network plane as public-facing endpoints. Segregating public services into dedicated subnets and limiting direct inbound access reduces attack surface, prevents easy lateral movement, and creates a clearer audit trail — all necessary to demonstrate compliance during assessments.
\n\nDesign to a “public DMZ + private backend” pattern: put load balancers or application gateways in a public subnet (with a minimal set of allowed ports), keep application servers and databases in private subnets without public IPs, and use managed NAT/egress paths for outbound access. Enforce least privilege at the network level with security groups/NSGs/firewall rules, log flows, and require authenticated management access via a bastion or session manager. For Compliance Framework practice, document the subnetwork design, explain how it limits exposure, and show evidence (route tables, firewall rules, logs) during an assessment.
\n\nIn AWS create a VPC with at least two public subnets (one per AZ) and two private subnets for app/db. Attach an Internet Gateway and associate route table entries for public subnets (0.0.0.0/0 -> igw-xxxx). For private subnets use AWS Cloud NAT (high-availability NAT Gateway across AZs) so instances have outbound internet without public IPs. Place an Application Load Balancer (ALB) in the public subnets; do NOT assign public IPs to backend EC2s/RDS. Use security groups: ALB SG allow inbound 80/443 from 0.0.0.0/0, backend SG allow inbound only from the ALB SG security group on the app port. Avoid SSH to public IPs — prefer AWS Systems Manager Session Manager or a bastion in a restricted management subnet; lock SSH by source IP. Enable VPC Flow Logs to CloudWatch or S3, enable CloudTrail, and turn on GuardDuty. For Terraform, use modules for aws_vpc, aws_subnet, aws_nat_gateway, aws_lb, aws_security_group to codify the design so you can show configuration as evidence.
\n\nIn Azure create a VNet with public/front-end subnet for an Application Gateway (or Public Load Balancer) and private backend subnets for VMs and PaaS resources. Use Azure Firewall or an NSG with explicit deny rules to control ingress/egress. Ensure VMs in private subnets have no public IPs; use Azure Bastion or Point-to-Site VPN for admin access. Use NAT Gateway for outbound connectivity from private subnets. Place WAF (Application Gateway WAF or Azure Front Door) in front of public endpoints for basic protection. Collect diagnostics and NSG flow logs via Network Watcher and forward logs to a Log Analytics workspace — these logs are compliance artifacts demonstrating enforcement and monitoring.
\n\nIn GCP create a VPC with separate subnets for the external load balancer and backend instances; use the external HTTPS load balancer (which terminates TLS at the edge) and avoid giving VMs external IPs. Use Cloud NAT so private instances can make outbound connections without external IPs. Apply Firewall rules to allow only the required ports (e.g., 443) to the load balancer target tags and block direct 0.0.0.0/0 to internal instances. Enable VPC Flow Logs, Cloud Audit Logs, and use Cloud Armor (WAF) and Cloud IDS for additional protection. Use IAM roles and service accounts with least privilege to manage network resources; document the network design and firewall rule set for audits.
\n\nExample: A small contractor hosts a customer-facing web app that displays only public content and collects non-sensitive contact forms. Implement a single public subnet with a managed HTTPS load balancer (ALB / Application Gateway / Cloud Load Balancing) and two private subnets for application and DB servers across AZs. Do not assign public IPs to app/database VMs. Use a managed database service in private subnets. Configure security group/NSG/firewall rules to accept only 443 at the load balancer and allow the load balancer to talk to the app tier on a specific port. Use SSM or Bastion + MFA for admin access, enable flow logs and TLS certificate management, and archive logs for the period your compliance policy requires. This is low-cost, scalable, and easy to demonstrate in an audit with route tables, SG/NSG rules, and logs as artifacts.
\n\nCompliance tips: document your network diagram and policies, codify configuration with IaC (Terraform/ARM/Bicep/Deployment Manager) to provide reproducible evidence, enable long-term log retention, and schedule periodic reviews of security group/firewall rules. Best practices include using managed load balancers and WAF, restricting management access by IP and tooling (SSM/Bastion), applying principle-of-least-privilege on service accounts, and running automated checks (Cloud Custodian, Azure Policy, Forseti/Policy Controller) to prevent drift. Risks of not implementing these controls: direct exposure of internal systems, increased probability of data breaches or compromise of CUI, loss of contract eligibility, potential financial penalties, and failure in CMMC/FAR assessments. Network misconfiguration is a common root cause of incidents — the extra effort to isolate public services mitigates this risk substantially.
\n\nIn summary, meeting FAR 52.204-21 and CMMC 2.0 Level 1 for public-facing services requires a disciplined network-level approach: segregate public endpoints into dedicated subnets, avoid public IPs on backend systems, enforce strict security group/NSG/firewall rules, use managed NAT and load balancing, enable logging/monitoring, and document everything. For small businesses, this can be implemented with mostly managed cloud services and a short list of controls to maintain and demonstrate during audit.
", "plain_text": "This post explains how to design and configure cloud VPC/VNet subnetworking for public-facing services in a way that satisfies the intent of FAR 52.204-21 and CMMC 2.0 Level 1 control SC.L1-B.1.XI — minimizing exposure of systems and data while enabling necessary internet access for public workloads.\n\nWhy subnetwork segregation matters for FAR 52.204-21 / CMMC Level 1\nFAR 52.204-21 and CMMC Level 1 require basic safeguarding of information and cyber hygiene; a core implementation pattern is to avoid placing sensitive data and internal services on the same network plane as public-facing endpoints. Segregating public services into dedicated subnets and limiting direct inbound access reduces attack surface, prevents easy lateral movement, and creates a clearer audit trail — all necessary to demonstrate compliance during assessments.\n\nHigh-level design principles and Compliance Framework implementation notes\nDesign to a “public DMZ + private backend” pattern: put load balancers or application gateways in a public subnet (with a minimal set of allowed ports), keep application servers and databases in private subnets without public IPs, and use managed NAT/egress paths for outbound access. Enforce least privilege at the network level with security groups/NSGs/firewall rules, log flows, and require authenticated management access via a bastion or session manager. For Compliance Framework practice, document the subnetwork design, explain how it limits exposure, and show evidence (route tables, firewall rules, logs) during an assessment.\n\nPractical implementation - AWS (step-by-step)\nIn AWS create a VPC with at least two public subnets (one per AZ) and two private subnets for app/db. Attach an Internet Gateway and associate route table entries for public subnets (0.0.0.0/0 -> igw-xxxx). For private subnets use AWS Cloud NAT (high-availability NAT Gateway across AZs) so instances have outbound internet without public IPs. Place an Application Load Balancer (ALB) in the public subnets; do NOT assign public IPs to backend EC2s/RDS. Use security groups: ALB SG allow inbound 80/443 from 0.0.0.0/0, backend SG allow inbound only from the ALB SG security group on the app port. Avoid SSH to public IPs — prefer AWS Systems Manager Session Manager or a bastion in a restricted management subnet; lock SSH by source IP. Enable VPC Flow Logs to CloudWatch or S3, enable CloudTrail, and turn on GuardDuty. For Terraform, use modules for aws_vpc, aws_subnet, aws_nat_gateway, aws_lb, aws_security_group to codify the design so you can show configuration as evidence.\n\nPractical implementation - Azure (step-by-step)\nIn Azure create a VNet with public/front-end subnet for an Application Gateway (or Public Load Balancer) and private backend subnets for VMs and PaaS resources. Use Azure Firewall or an NSG with explicit deny rules to control ingress/egress. Ensure VMs in private subnets have no public IPs; use Azure Bastion or Point-to-Site VPN for admin access. Use NAT Gateway for outbound connectivity from private subnets. Place WAF (Application Gateway WAF or Azure Front Door) in front of public endpoints for basic protection. Collect diagnostics and NSG flow logs via Network Watcher and forward logs to a Log Analytics workspace — these logs are compliance artifacts demonstrating enforcement and monitoring.\n\nPractical implementation - GCP (step-by-step)\nIn GCP create a VPC with separate subnets for the external load balancer and backend instances; use the external HTTPS load balancer (which terminates TLS at the edge) and avoid giving VMs external IPs. Use Cloud NAT so private instances can make outbound connections without external IPs. Apply Firewall rules to allow only the required ports (e.g., 443) to the load balancer target tags and block direct 0.0.0.0/0 to internal instances. Enable VPC Flow Logs, Cloud Audit Logs, and use Cloud Armor (WAF) and Cloud IDS for additional protection. Use IAM roles and service accounts with least privilege to manage network resources; document the network design and firewall rule set for audits.\n\nSmall-business real-world scenario and practical checklist\nExample: A small contractor hosts a customer-facing web app that displays only public content and collects non-sensitive contact forms. Implement a single public subnet with a managed HTTPS load balancer (ALB / Application Gateway / Cloud Load Balancing) and two private subnets for application and DB servers across AZs. Do not assign public IPs to app/database VMs. Use a managed database service in private subnets. Configure security group/NSG/firewall rules to accept only 443 at the load balancer and allow the load balancer to talk to the app tier on a specific port. Use SSM or Bastion + MFA for admin access, enable flow logs and TLS certificate management, and archive logs for the period your compliance policy requires. This is low-cost, scalable, and easy to demonstrate in an audit with route tables, SG/NSG rules, and logs as artifacts.\n\nCompliance tips, best practices, and risks of not implementing\nCompliance tips: document your network diagram and policies, codify configuration with IaC (Terraform/ARM/Bicep/Deployment Manager) to provide reproducible evidence, enable long-term log retention, and schedule periodic reviews of security group/firewall rules. Best practices include using managed load balancers and WAF, restricting management access by IP and tooling (SSM/Bastion), applying principle-of-least-privilege on service accounts, and running automated checks (Cloud Custodian, Azure Policy, Forseti/Policy Controller) to prevent drift. Risks of not implementing these controls: direct exposure of internal systems, increased probability of data breaches or compromise of CUI, loss of contract eligibility, potential financial penalties, and failure in CMMC/FAR assessments. Network misconfiguration is a common root cause of incidents — the extra effort to isolate public services mitigates this risk substantially.\n\nIn summary, meeting FAR 52.204-21 and CMMC 2.0 Level 1 for public-facing services requires a disciplined network-level approach: segregate public endpoints into dedicated subnets, avoid public IPs on backend systems, enforce strict security group/NSG/firewall rules, use managed NAT and load balancing, enable logging/monitoring, and document everything. For small businesses, this can be implemented with mostly managed cloud services and a short list of controls to maintain and demonstrate during audit." }, "metadata": { "description": "Step-by-step guidance to architect and configure public-facing cloud subnetworks so small businesses can meet FAR 52.204-21 and CMMC 2.0 Level 1 network isolation and minimal exposure requirements.", "permalink": "/how-to-configure-cloud-vpc-subnetworks-for-public-facing-services-to-satisfy-far-52204-21-cmmc-20-level-1-control-scl1-b1xi-awsazuregcp.json", "categories": [], "tags": [] } }