This post provides a practical, vendor-neutral playbook for implementing subnetworks for public-facing components (web servers, APIs, reverse proxies) to satisfy the intent of FAR 52.204-21 and CMMC 2.0 Level 1 control SC.L1-B.1.XI: isolate public-facing systems from internal networks using subnetworks and network controls that reduce exposure of Controlled Unclassified Information (CUI) and limit lateral movement.
\n\nStart by cataloging every service that must be reachable from the internet (web servers, API endpoints, public file shares, third-party integrations). For each item record: purpose, data classification (does it touch CUI?), protocols/ports, expected IP/URL, owner, and maintenance windows. For a small business this can be a simple spreadsheet or an asset inventory in your ticketing system. This classification determines whether a component truly needs a public-facing IP or can be placed behind an application delivery layer (load balancer/reverse proxy).
\n\nDecide whether you will do cloud VPC/VNet subnets, on-prem VLANs, or a hybrid design; keep it simple for small orgs. Example CIDR plan for an AWS-like VPC: 10.0.0.0/16 with public subnets 10.0.1.0/24, private app subnets 10.0.2.0/24, DB subnets 10.0.3.0/24. Allocate a dedicated public-subnet range per AZ or physical location. Document route tables: public subnets route 0.0.0.0/0 → Internet Gateway; private subnets route to NAT gateway/instances for egress. For on-prem VLANs, map VLAN IDs (e.g., VLAN 10: Public DMZ; VLAN 20: Internal) and assign IP pools to match your routing/firewall plan.
\n\nPlace only the minimum required components in public subnets — ideally just a reverse proxy/load balancer, NAT gateways, and jump/bastion hosts. Avoid putting application servers or databases directly into public subnets. In cloud environments, tag public subnets clearly (e.g., \"env:prod, role:public-subnet\") and attach a specific route table that only allows Internet Gateway connectivity for the services that need it. On-prem, enforce with switch port profiles and ACLs on the distribution/edge switches.
\n\nApply defense-in-depth at the network edge: use a perimeter firewall (or cloud security groups + NACLs), put a load balancer or reverse proxy in the public subnet, and protect apps with a WAF when handling HTTP/S. For small teams, use managed services (AWS ALB + WAF, Azure Application Gateway + WAF) to simplify operations. Example access rules: allow inbound 80/443 to the load balancer from 0.0.0.0/0, allow backend traffic from the load balancer's security group to the private app subnet on required ports, block direct inbound SSH/RDP from internet (only allow via bastion or VPN). Below are two compact diagram examples — include these in documentation and audit artifacts.
\n\n\nCloud (AWS-like) example:\nInternet\n |\n[IGW] — [Public Subnet: ALB/WAF (10.0.1.0/24)] <- only ALB has public IPs\n |\n [Private Subnet: App Servers (10.0.2.0/24)] <- no direct IGW route\n |\n [DB Subnet: DB Servers (10.0.3.0/24)]\n\nOn-prem example:\nInternet -> Perimeter Firewall -> DMZ VLAN (VLAN 10, 192.168.10.0/24): reverse proxy/load balancer\n -> Internal VLAN (VLAN 20, 192.168.20.0/24): app servers\n -> Management VLAN (VLAN 99): VPN/bastion for admin access\n\n\n
Create allowlists, not broad allow-any rules. Example firewall/security-group rules: Inbound: ALB: TCP 80/443 from 0.0.0.0/0; Backend servers: TCP 443 from ALB SG only; Management bastion: SSH (22) or RDP (3389) only from your corporate VPN/network public IP ranges, or better, use ephemeral systems manager (AWS SSM) or jump-host over an IPsec/SSL VPN. For stateless NACLs add explicit deny rules for suspicious ports (e.g., block NetBIOS, SMB 445). Keep a template of rules you can reuse across environments — this evidence is useful during compliance reviews.
\n\nEnable VPC Flow Logs / NSG flow logs / firewall logging and forward to a central log store or SIEM (Splunk, Elastic, or a managed service). Create alerts for anomalous public traffic patterns (excessive failed connections to app servers, connections from blacklisted ASNs). Automate deployment of subnets and security groups via IaC (Terraform, ARM/Bicep, CloudFormation) so configurations are reproducible and auditable. For small-business budgets, scheduled exports to cloud object storage and lightweight analytics (Lambda functions or scheduled scripts) can produce the required evidence for continuous monitoring and audits.
\n\nConduct targeted tests: external port scans, web app scans, and internal lateral-movement checks. Maintain a change log that ties config changes to tickets and approvals. Produce documentation for auditors that includes: asset inventory, topology diagrams (like the ASCII examples above), configured CIDR blocks, route tables, firewall/security-group rule exports, IAM/service account policies used by public components, and logging retention settings. For CMMC/FAR adherence, show that public components are segregated and only necessary ports/services are exposed, and that there is a process for review and patching.
\n\nFailing to isolate public components significantly increases attack surface and risk of lateral movement into sensitive systems that hold CUI. Direct exposure of application servers or management interfaces can lead to data breaches, ransomware, and loss of DoD contracts or federal work if noncompliant with FAR 52.204-21 or CMMC requirements. Operational risks include longer incident response times, greater blast radius for vulnerabilities, and difficulty producing audit evidence — all of which can jeopardize small-business continuity and reputation.
\n\nImplementing subnetworks for public components is a practical and achievable control for small businesses seeking FAR and CMMC compliance: inventory what is public, design simple CIDR-based subnets, place only gateway components in public subnets, enforce strict firewall/security-group rules, centralize logging, and automate and document changes. Quick wins include: disabling direct management from the internet, using a bastion or managed access service, enabling flow logs, and codifying the network design in IaC. These steps reduce exposure, simplify audits, and align with the intent of Control SC.L1-B.1.XI and FAR 52.204-21 while remaining operationally manageable for small teams.
", "plain_text": "This post provides a practical, vendor-neutral playbook for implementing subnetworks for public-facing components (web servers, APIs, reverse proxies) to satisfy the intent of FAR 52.204-21 and CMMC 2.0 Level 1 control SC.L1-B.1.XI: isolate public-facing systems from internal networks using subnetworks and network controls that reduce exposure of Controlled Unclassified Information (CUI) and limit lateral movement.\n\n7 Practical Steps to Implement Subnetworks for Public Components\n\nStep 1 — Inventory and classify public-facing components\nStart by cataloging every service that must be reachable from the internet (web servers, API endpoints, public file shares, third-party integrations). For each item record: purpose, data classification (does it touch CUI?), protocols/ports, expected IP/URL, owner, and maintenance windows. For a small business this can be a simple spreadsheet or an asset inventory in your ticketing system. This classification determines whether a component truly needs a public-facing IP or can be placed behind an application delivery layer (load balancer/reverse proxy).\n\nStep 2 — Choose a topology and address plan\nDecide whether you will do cloud VPC/VNet subnets, on-prem VLANs, or a hybrid design; keep it simple for small orgs. Example CIDR plan for an AWS-like VPC: 10.0.0.0/16 with public subnets 10.0.1.0/24, private app subnets 10.0.2.0/24, DB subnets 10.0.3.0/24. Allocate a dedicated public-subnet range per AZ or physical location. Document route tables: public subnets route 0.0.0.0/0 → Internet Gateway; private subnets route to NAT gateway/instances for egress. For on-prem VLANs, map VLAN IDs (e.g., VLAN 10: Public DMZ; VLAN 20: Internal) and assign IP pools to match your routing/firewall plan.\n\nStep 3 — Create dedicated public subnet(s) and restrict placement\nPlace only the minimum required components in public subnets — ideally just a reverse proxy/load balancer, NAT gateways, and jump/bastion hosts. Avoid putting application servers or databases directly into public subnets. In cloud environments, tag public subnets clearly (e.g., \"env:prod, role:public-subnet\") and attach a specific route table that only allows Internet Gateway connectivity for the services that need it. On-prem, enforce with switch port profiles and ACLs on the distribution/edge switches.\n\nStep 4 — Implement perimeter controls (firewall, LB, WAF) and show network diagrams\nApply defense-in-depth at the network edge: use a perimeter firewall (or cloud security groups + NACLs), put a load balancer or reverse proxy in the public subnet, and protect apps with a WAF when handling HTTP/S. For small teams, use managed services (AWS ALB + WAF, Azure Application Gateway + WAF) to simplify operations. Example access rules: allow inbound 80/443 to the load balancer from 0.0.0.0/0, allow backend traffic from the load balancer's security group to the private app subnet on required ports, block direct inbound SSH/RDP from internet (only allow via bastion or VPN). Below are two compact diagram examples — include these in documentation and audit artifacts.\n\n\nCloud (AWS-like) example:\nInternet\n |\n[IGW] — [Public Subnet: ALB/WAF (10.0.1.0/24)] Perimeter Firewall -> DMZ VLAN (VLAN 10, 192.168.10.0/24): reverse proxy/load balancer\n -> Internal VLAN (VLAN 20, 192.168.20.0/24): app servers\n -> Management VLAN (VLAN 99): VPN/bastion for admin access\n\n\nStep 5 — Define explicit, minimal network rules and management access\nCreate allowlists, not broad allow-any rules. Example firewall/security-group rules: Inbound: ALB: TCP 80/443 from 0.0.0.0/0; Backend servers: TCP 443 from ALB SG only; Management bastion: SSH (22) or RDP (3389) only from your corporate VPN/network public IP ranges, or better, use ephemeral systems manager (AWS SSM) or jump-host over an IPsec/SSL VPN. For stateless NACLs add explicit deny rules for suspicious ports (e.g., block NetBIOS, SMB 445). Keep a template of rules you can reuse across environments — this evidence is useful during compliance reviews.\n\nStep 6 — Logging, monitoring, and automation\nEnable VPC Flow Logs / NSG flow logs / firewall logging and forward to a central log store or SIEM (Splunk, Elastic, or a managed service). Create alerts for anomalous public traffic patterns (excessive failed connections to app servers, connections from blacklisted ASNs). Automate deployment of subnets and security groups via IaC (Terraform, ARM/Bicep, CloudFormation) so configurations are reproducible and auditable. For small-business budgets, scheduled exports to cloud object storage and lightweight analytics (Lambda functions or scheduled scripts) can produce the required evidence for continuous monitoring and audits.\n\nStep 7 — Test, document, and show compliance evidence\nConduct targeted tests: external port scans, web app scans, and internal lateral-movement checks. Maintain a change log that ties config changes to tickets and approvals. Produce documentation for auditors that includes: asset inventory, topology diagrams (like the ASCII examples above), configured CIDR blocks, route tables, firewall/security-group rule exports, IAM/service account policies used by public components, and logging retention settings. For CMMC/FAR adherence, show that public components are segregated and only necessary ports/services are exposed, and that there is a process for review and patching.\n\nRisk of not implementing subnetworks for public components\nFailing to isolate public components significantly increases attack surface and risk of lateral movement into sensitive systems that hold CUI. Direct exposure of application servers or management interfaces can lead to data breaches, ransomware, and loss of DoD contracts or federal work if noncompliant with FAR 52.204-21 or CMMC requirements. Operational risks include longer incident response times, greater blast radius for vulnerabilities, and difficulty producing audit evidence — all of which can jeopardize small-business continuity and reputation.\n\nSummary and compliance tips\nImplementing subnetworks for public components is a practical and achievable control for small businesses seeking FAR and CMMC compliance: inventory what is public, design simple CIDR-based subnets, place only gateway components in public subnets, enforce strict firewall/security-group rules, centralize logging, and automate and document changes. Quick wins include: disabling direct management from the internet, using a bastion or managed access service, enabling flow logs, and codifying the network design in IaC. These steps reduce exposure, simplify audits, and align with the intent of Control SC.L1-B.1.XI and FAR 52.204-21 while remaining operationally manageable for small teams." }, "metadata": { "description": "Practical, step-by-step guidance for creating isolated subnetworks (DMZ/public subnets) to meet FAR 52.204-21 and CMMC 2.0 Level 1 network segregation expectations while minimizing risk for small businesses.", "permalink": "/implementing-subnetworks-for-public-components-7-practical-steps-and-network-diagram-examples-far-52204-21-cmmc-20-level-1-control-scl1-b1xi.json", "categories": [], "tags": [] } }