CMMC 3.13.5 (Public Subnetwork Separation): Evidence & What Assessors Actually Check
Control: NIST SP 800-171 Rev. 2 §3.13.5 · CMMC Practice: SC.L2-3.13.5 · Updated June 2026
Plain-English answer
3.13.5 requires you to put publicly accessible system components in subnetworks physically or logically separated from your internal networks — a DMZ. Two steps: identify what's internet-facing, then make sure it doesn't share a network segment with your internal systems and CUI.
Who this applies to
Every organization seeking CMMC Level 2 for a CUI environment — most directly anyone hosting internet-facing services (web, mail relay, VPN, remote-access gateways).
Why it matters
Public-facing systems get attacked first. Separating them means a compromised web server can't pivot straight onto the network holding your CUI. It carries a weighted SPRS deduction if unmet (verify the exact value against the DoD Assessment Methodology).
Control lineage
- CMMC Practice: SC.L2-3.13.5
- NIST 800-171 Rev. 2: §3.13.5 — implement subnetworks for publicly accessible system components that are physically or logically separated from internal networks.
- NIST 800-171A objectives: [a] publicly accessible system components are identified; [b] subnetworks for publicly accessible system components are physically or logically separated from internal networks.
What the assessor will EXAMINE
- A list of publicly accessible system components (or a documented "none").
- Network diagram showing those components in a separated subnetwork / DMZ.
- Firewall/VLAN configuration enforcing the separation between the DMZ and internal networks.
- SSP narrative for 3.13.5.
What the assessor will INTERVIEW & TEST
- Interview: "What's publicly accessible, and how is it separated from your internal network?" — point to the inventory + diagram + segmentation rules.
- Test: Confirm the DMZ segment can't freely initiate connections into the internal network.
- Test: Reconcile the public-component list against what's actually internet-reachable (no surprise exposed hosts).
Evidence examples (produce these)
- Inventory of publicly accessible components (or documented "none applicable" with rationale).
- Network diagram showing DMZ separation from internal networks.
- Firewall/VLAN/ACL config enforcing DMZ-to-internal restrictions.
- SSP section 3.13.5 referencing the above.
Common failure patterns
- A public web/mail server on the same flat LAN as internal systems — objective [b] fails.
- Public components never inventoried, so exposure is unknown — objective [a] fails.
- A "DMZ" that can still reach internal hosts at will — separation in name only.
- Shadow internet-facing services (a forgotten port-forward) not in the inventory.
SPRS impact & POA&M
SPRS: 3.13.5 carries a weighted deduction (verify the exact value). POA&M: confirm 3.13.5's POA&M eligibility against current CMMC rules before assuming it can be deferred.
Assessor-ready summary
You're good on 3.13.5 when every publicly accessible component is identified and sits in a subnetwork physically or logically separated from internal networks, the separation is enforced by configuration (not just drawn on a diagram), and the SSP matches reality.
Is anything internet-facing sharing a segment with your CUI? Get a segmentation evidence map →
Related: 3.13.1 Boundary Protection · 3.1.20 External Systems · All control pages
Athena prepares organizations for CMMC Level 2; certification rests with C3PAOs/DIBCAC. Verify all control facts against NIST SP 800-171 Rev. 2, NIST SP 800-171A, and current CMMC rules.