Implementing isolated subnetworks in an AWS VPC is a core networking control to protect Federal Contract Information (FCI) and meet requirements such as FAR 52.204-21 and CMMC 2.0 Level 1 (SC.L1‑B.1.XI); this post gives a practical, small‑business focused implementation pattern, concrete AWS configuration steps, and compliance evidence you can produce during audits.
Compliance context: why isolation maps to FAR 52.204-21 and CMMC 2.0 Level 1
FAR 52.204-21 requires contractors to provide basic safeguarding of contractor information systems, and CMMC 2.0 Level 1 includes basic cyber hygiene practices to limit access to Federal Contract Information. Network segmentation and isolated subnetworks reduce the attack surface and limit the lateral movement of unauthorized actors—both key objectives for these standards. For an auditor, isolation demonstrates technical capability to protect FCI by separating sensitive workloads from internet‑facing or less‑trusted resources.
Architectural pattern: isolated subnets in an AWS VPC
VPC layout (example CIDR and subnet plan)
A simple, repeatable VPC layout for a small business: one VPC per environment (prod/stage/dev) using 10.0.0.0/16. Create three logical tiers: public (10.0.1.0/24) for load balancers and bastion hosts, private with egress (10.0.2.0/24) for application servers that need outbound updates, and isolated/private without internet (10.0.3.0/24) for systems handling FCI (databases, file stores). Example CLI to create the VPC: aws ec2 create-vpc --cidr-block 10.0.0.0/16 --tag-specifications 'ResourceType=vpc,Tags=[{Key=Name,Value=fcivpc}]'. Tag subnets with "Environment", "Tier", and "Compliance=FCI".
Routing, Internet/NAT, and VPC endpoints
Associate separate route tables per subnet tier. Public subnets have a route to an Internet Gateway (IGW); private subnets that require controlled outbound access use a NAT Gateway in a public subnet and a route 0.0.0.0/0 → nat-xxxx. For truly isolated subnets handling FCI, do NOT add a route to an IGW or NAT—remove any 0.0.0.0/0 route. Instead use VPC Gateway and Interface Endpoints (e.g., S3 gateway endpoint, com.amazonaws.
Security Groups, Network ACLs, and IAM controls
Use security groups as primary host‑level firewalls (stateful) allowing only required service ports and source CIDRs. Use NACLs as a secondary, stateless filter to block broad unwanted traffic between subnets. Enforce least privilege in IAM: deny ability for developers to create or attach an Internet Gateway or to modify route tables for compliance subnets; create an IAM permission boundary or SCP in AWS Organizations to prevent accidental topology changes. Enable VPC Flow Logs and send them to a centralized CloudWatch Log Group or S3 bucket with restricted access to provide evidence of traffic flows.
Practical implementation steps for a small business
1) Design the VPC CIDR and subnet map and document it in your System Security Plan (SSP). 2) Automate deployment with Terraform or CloudFormation to ensure repeatability and auditable templates—example Terraform modules: vpc, subnet, route_table, nat_gateway, vpc_endpoint. 3) Apply tagging and enforce with AWS Config rules (e.g., required-tags, restricted-public-ip). 4) Harden host images and attach roles with least privilege for service-to-service access. 5) Deploy monitoring (CloudWatch Alarms, GuardDuty, Security Hub) and enable CloudTrail across all regions for immutable audit logs. These steps create evidence you can present for FAR/CMMC assessments: IaC templates, tag inventories, and logs showing no egress from FCI subnets.
Compliance tips, monitoring, and evidence collection
For auditors, produce: the VPC architecture diagram, the IaC templates with commit history, AWS Config snapshots showing route table associations, and VPC Flow Logs demonstrating no internet egress from isolated subnets. Implement AWS Config managed rules like vpc-default-security-group-closed and custom rules that enforce "no route to IGW or NAT" for subnets tagged Compliance=FCI. Retain logs for the period required by contract (commonly 1–3 years) and document change control approvals for any topology changes involving compliance subnets.
Real-world small business scenario
Example: a small defense subcontractor hosts R&D documents containing FCI in an S3 bucket and runs an internal wiki on EC2. They create an isolated subnet for RDS and an EC2 file-processing server without any IGW/NAT route, and add an S3 gateway endpoint in the VPC so the file server can reach S3 directly. Public access is limited to an ALB in the public subnets that proxies requests to application servers in private-with-egress subnets; only the ALB and a secured bastion have public IPs. This pattern allowed them to pass a CMMC Level 1 assessment by showing that FCI storage and processing never traverse the public internet from their VPC.
Risk of not implementing isolated subnetworks
Without network isolation, FCI and other sensitive data can be exposed via unintended internet egress, misconfigured instances, or lateral movement after compromise. Risks include data exfiltration, contract termination, financial penalties, loss of future government work, and reputational damage. Technically, a public-facing misconfigured server could pivot and access databases that should have been isolated—segmentation prevents or contains such incidents.
Summary: For FAR 52.204-21 and CMMC 2.0 Level 1 (SC.L1‑B.1.XI) compliance, implementing isolated subnetworks in AWS using a clear VPC design, strict route table policies, VPC endpoints, security groups, NACLs, IAM restrictions, and centralized logging provides both technical protection and the evidence auditors expect; automate the setup, enforce tags and config rules, collect logs, and document the architecture to make compliance repeatable and defensible.
