Logical subnet separation in AWS is a practical, low-cost control you can implement to help meet FAR 52.204-21 and the CMMC 2.0 Level 1 objective SC.L1-B.1.XI: limit internal network access and separate system components so that sensitive Federal Contract Information (FCI) and business-critical systems are protected from unnecessary exposure and lateral movement.
Compliance Framework context and key objectives
Framework: Compliance Framework. Practice: Practice. Requirement: implement network segmentation or logical separation to restrict access between components that do not need to communicate directly. Key objectives: minimize attack surface, enforce least privilege at the network layer, create clear audit trails, and provide demonstrable evidence (architecture diagrams, configuration snapshots, logs) for auditors. Implementation Notes: design VPCs and subnets to reflect trust boundaries, use security groups and NACLs for enforcement, leverage AWS managed services (VPC Flow Logs, CloudTrail, AWS Config) to collect evidence and detect violations.
Design patterns for logical subnet separation in AWS
Start with a VPC and divide it into well-named subnets that map to functional tiers: public (load balancers), application (app servers), data (databases), and management (bastion/ops). Use non-overlapping CIDR ranges (for example, VPC 10.0.0.0/16 with /24 subnets: 10.0.1.0/24 public, 10.0.2.0/24 app, 10.0.3.0/24 db, 10.0.4.0/24 mgmt). Apply a route table per tier: public subnets route to an Internet Gateway (IGW), private subnets have no IGW and use a NAT Gateway for outbound updates, and database subnets have no direct internet routes. This gives you a predictable network topology that maps directly to compliance requirements.
Network controls: security groups, NACLs and route tables
Security Groups (SGs) are stateful and should be your primary enforcement for instance-to-instance rules—use SG references to allow traffic only from specific SGs (for example, DB SG allows TCP/5432 only from App SG). Network ACLs (NACLs) are stateless and operate at the subnet level; use them for broad subnet-level policies (e.g., deny known malicious ranges or block all inbound except expected ephemeral ports). Keep route tables simple: do not attach an IGW to DB subnets, and avoid unnecessary routing between subnets. Example rules: ALB SG — allow TCP 443 from 0.0.0.0/0; App SG — allow TCP 443 from ALB SG; DB SG — allow TCP 5432 from App SG only; Management SG — allow TCP 22 (or better, use SSM Session Manager) from the company's admin CIDR only.
Supporting services and secure management
Prefer AWS-managed paths for sensitive services: use VPC Gateway Endpoints for S3 (no public egress), Interface Endpoints (PrivateLink) for API calls to AWS services, and Systems Manager (SSM) Session Manager to manage EC2 without SSH/RDP exposure. Use IAM roles for instances instead of embedding credentials. For outbound OS updates from private subnets, use a NAT Gateway in a controlled subnet or an EC2 proxy in a management subnet and restrict its traffic. Implement a dedicated management subnet that has the least connectivity to production tiers and is monitored more closely.
Small business real-world scenario
Example: a small defense contractor runs a customer portal (ALB -> ECS/EKS -> RDS). Implement three subnets across two AZs: public for ALB, private-app for containers, private-db for RDS. Configure SGs so containers accept traffic only from the ALB SG and RDS only from the app SG. Disable public IPs on containers and RDS. Use SSM for patching and configuration so SSH is not open to the internet. Collect evidence by exporting the VPC flow logs and SG snapshots before and after deployment and store them in a locked S3 bucket with MFA Delete enabled (or follow your organization's retention). Use Terraform or CloudFormation to template the architecture so the small IT team can reproduce and show versioned infrastructure to an auditor.
Operational monitoring, automation, and evidence collection
Operationalize the design: enable VPC Flow Logs and send to CloudWatch Logs or an S3 bucket; enable AWS Config and enable managed rules such as vpc-flow-logs-enabled, restricted-common-ports, and s3-bucket-public-read-prohibited. Use Security Hub to aggregate findings and set up automated remediation (e.g., Lambda that flags or remediates overly permissive SGs). For audits, produce an architecture diagram, the CloudFormation/Terraform state file, security group rule exports, and a sample of VPC Flow Logs that show only expected traffic flows. Document change-control tickets for any modification to subnet or SG layouts.
Risk of not implementing logical subnet separation includes increased lateral movement risk (an attacker who compromises a web tier can reach a database tier), higher chance of data exfiltration, inability to produce evidence showing separation for FAR/CMMC assessments, and potential contract or certification loss. Small businesses often underestimate how quickly permissive security groups escalate a minor compromise into a major breach.
In summary, logical subnet separation in AWS to satisfy FAR 52.204-21 and CMMC 2.0 Level 1 is achieved through clear VPC/subnet planning, strict security group and NACL configurations, limited routing and use of VPC endpoints, and operational controls like VPC Flow Logs, AWS Config, and IAM role usage. Use templated infrastructure, documented evidence, and automated checks so you can demonstrate compliance and reduce risk without adding undue operational overhead.
