Isolating public-facing services into dedicated subnetworks — whether in the cloud or on-premises — is a practical control to reduce attack surface and meet the basic safeguarding requirements of FAR 52.204-21 and the CMMC 2.0 Level 1 control SC.L1-B.1.XI; this post provides a hands-on implementation checklist, technical examples for common platforms, and small-business scenarios you can use to demonstrate compliance.
What the Control Requires and Why It Matters
At a high level, FAR 52.204-21 requires basic safeguarding of Federal Contract Information (FCI). CMMC 2.0 Level 1 mirrors that by mandating basic cyber hygiene controls; SC.L1-B.1.XI specifically targets the isolation of public-facing services so that systems processing or storing FCI are not unnecessarily exposed. Practically, that means deploying public-facing workloads in segregated subnetworks/DMZs, enforcing strict ingress/egress rules, and ensuring internal systems (databases, file shares, admin consoles) are reachable only through controlled, logged paths.
Design Patterns: Cloud and On-Prem Architectures
Cloud: VPC/VNet segmentation, NAT, ALB/WAF, and security groups/NSGs
In cloud environments (AWS/Azure/GCP) implement a two-tier subnet model inside your VPC/VNet: public subnets for load balancers, reverse proxies, or edge services and private subnets for application servers and data stores. Example AWS setup: VPC 10.0.0.0/16, public subnet 10.0.1.0/24 (ALB/NGINX reverse proxy), private subnet 10.0.2.0/24 (app servers), DB subnet 10.0.3.0/24 (RDS). Attach an Internet Gateway to the VPC, configure a route table so only the public subnet routes 0.0.0.0/0 to the IGW, and use a NAT Gateway in the public subnet to allow private subnets outbound internet access for patching. Enforce inbound rules: ALB SG allows TCP 443 from 0.0.0.0/0; app SG allows 443/8080 only from ALB SG; DB SG allows 5432 or 3306 only from app SG. Complement security groups with NACLs (stateless) for additional defense-in-depth and a WAF in front of your ALB to block common web attacks.
On-Prem and Hybrid: VLANs, DMZs, firewalls, and VPN/Direct Connect
For on-prem, use VLANs or physically separate switches to create a DMZ VLAN for public-facing servers and private VLANs for internal systems. Example IPs: DMZ 192.168.10.0/24, internal apps 192.168.20.0/24, DBs 192.168.30.0/24. Place an edge firewall (pfSense, Palo Alto, Fortinet) between the internet and the DMZ; only allow inbound HTTP/HTTPS to the DMZ IPs and restrict outbound NAT from the DMZ. If you have hybrid connectivity to cloud, keep segmentation consistent across the tunnel: advertise only necessary prefixes and enforce route filters so the public DMZ subnets are not routable into internal-only networks without firewall review. Use reverse proxies and a WAF appliance where available and limit admin access to a management VLAN with MFA and jump hosts.
Implementation Checklist (Actionable Steps)
1) Identify and classify public-facing workloads and whether they handle FCI; 2) Design network topology (VPC/VNet + public/private/DB subnets or on-prem DMZ + VLANs); 3) Allocate CIDR ranges and route tables so public subnets are the only ones with IGW/Internet routes; 4) Deploy edge controls (ALB/Application Gateway + WAF, on-prem firewall + reverse proxy); 5) Implement least-privilege security group/NSG/NACL rules (allow only required ports and sources); 6) Deploy NAT for private subnet outbound; 7) Enforce admin access via bastion/jump host with MFA and just-in-time access; 8) Enable logging (flow logs, CloudTrail, firewall logs) and retain per policy; 9) Automate deployment with IaC (Terraform/ARM/AWS CloudFormation) and keep templates in version control; 10) Test segmentation with internal scans and a periodic penetration test and document results for audit evidence.
Small-Business Real-World Examples
Example A — Small contractor hosting a public marketing site and a client portal in AWS: use a single VPC with an ALB in public subnets and web app instances in private subnets; RDS in a DB subnet with no public IP. Configure ALB listener for 443 and terminate TLS at the ALB (use ACM-managed certificates). Enable AWS WAF to block SQLi/OWASP Top 10 patterns and turn on VPC Flow Logs + CloudWatch to capture traffic for 90 days. Example B — Small office with on-prem web app and an externally-facing API: place the webserver in a DMZ behind a pfSense box, NAT external port 443 to the DMZ IP, and restrict outbound to approved update servers; use VLAN tagging on a managed switch to separate DMZ and internal networks and require SSH access to admin consoles only from a dedicated management VLAN through a jump host.
Evidence, Risk, and Compliance Tips
For FAR 52.204-21 and CMMC Level 1 evidence, retain architecture diagrams showing subnets; export security group/NSG rules and firewall ACLs; keep logs (flow logs, firewall logs, access logs) and patch/update records; note change control and IaC templates; document training for staff managing the environment. Risks of not implementing proper subnet isolation include unauthorized access to internal systems, lateral movement after a compromise of a public server, disclosure of FCI, contract penalties, lost revenue, and reputational harm. Practical tips: adopt deny-by-default firewall rules, centralize logs to an inexpensive SIEM or log store (e.g., OSS ELK, managed CloudWatch Logs), enable automated patching where safe, and enforce MFA for all management interfaces. For a small business, prefer managed services (RDS, ALB, WAF) to reduce operational burden and provide stronger, auditable controls out of the box.
Summary: Segregating public-facing services into dedicated subnetworks is a straightforward, high-impact control that aligns with FAR 52.204-21 and CMMC 2.0 Level 1 SC.L1-B.1.XI — implement it by designing clear public/private subnet boundaries, enforcing least-privilege network rules, placing WAF and edge controls in front of public services, logging and documenting everything, and testing segmentation periodically; these steps both reduce risk and produce the artifacts auditors expect from a compliant small business.
