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NIST SP 800-172 Enhanced CUI Protection Center

03.13.15E — Separate Subnets to Isolate System Components and Functions

Read the official CUI requirement and assessment content, then use the separately labeled Bare Metal Cyber perspective to connect it to implementation, evidence, and sustained operation.

2Parameters
1Source controls
1Assessment objectives
3Assessment methods

03.13 — System and Communications Protection · NIST SP 800-172 Revision 3

Active
Official NIST requirement content

Security requirement

Implement [Organization-defined: SELECTED PARAMETER VALUE] separate subnetworks to isolate the following critical system components and functions: [Organization-defined: critical system components and functions].

Official NIST discussion

Discussion

Separating critical system components and functions from other noncritical system components and functions through separate subnetworks may be necessary to reduce susceptibility to a catastrophic or debilitating breach or compromise that results in system failure. For example, physically separating the command-and-control function from the in-flight entertainment function through separate subnetworks in a commercial aircraft provides an increased level of assurance in the trustworthiness of critical system functions. This requirement enhances SP 800-171 requirement 03.13.01.

Official organization-defined parameters

Tailoring decisions required

Resolve these values through the governing organization’s approved tailoring and risk-management process before declaring the requirement implemented.

SELECTED PARAMETER VALUE
critical system components and functionsorganization-defined critical system components and functionscritical system components and functions to be isolated are defined.
Official protection strategy classifications

Protection strategies supported

These classifications are carried from the official SP 800-172 OSCAL record and help explain the enhanced requirement’s defensive purpose.

PS-PRAPenetration-Resistant Architecture
Bare Metal Cyber interpretation

Implementation perspective

Treat Separate Subnets to Isolate System Components and Functions as an enhanced CUI protection outcome for elevated threat conditions that must be reflected in the system boundary, documented implementation, operational behavior, and assessment evidence. Pay particular attention to secure architecture, boundary protection, communications security, cryptography, and isolation of CUI processing.

  1. Confirm the federal agency selected this enhanced requirement for the critical program or high-value asset and identify the CUI system components, services, users, and external connections in scope.
  2. Resolve every organization-defined parameter through an approved governance and tailoring process.
  3. Map each clause of the requirement to an accountable owner, implementation mechanism, and evidence source.
  4. Verify that inherited and shared implementations are supported by current provider evidence and responsibility boundaries.
  5. Collect evidence during normal operation and review changes, exceptions, and deficiencies on a risk-based cadence.

Questions to ask

  • Which critical-program or high-value-asset CUI, data flows, users, and services are protected by this enhanced requirement?
  • Which portions are implemented locally, inherited, shared, or not applicable, and what evidence supports that determination?
  • Do the system security plan, deployed configuration, operating process, and assessment evidence tell the same story?
  • What change, incident, or threshold should trigger reassessment?

Evidence and validation

  • network and trust-boundary diagrams
  • firewall and gateway configurations
  • cryptographic configuration and key records
  • segmentation and isolation test results

Common failure patterns

  • diagrams omitting cloud and third-party paths
  • encryption enabled without key governance
  • flat trust zones permitting unnecessary lateral movement
  • boundary rules accumulating without owner review
Official NIST SP 800-172A content

Assessment objectives and methods

Assessment objectives (1)

  1. subnetworks are separated [Organization-defined: SELECTED PARAMETER VALUE] to isolate [Organization-defined: critical system components and functions].

Examine

  • System and communications protection policy
  • procedures addressing boundary protection
  • system design documentation
  • system hardware and software
  • system architecture
  • system configuration settings and associated documentation
  • criticality analysis
  • system audit records
  • system security plan
  • other relevant documents or records

Interview

  • System/network administrators
  • organizational personnel with information security responsibilities
  • system developer
  • organizational personnel with boundary protection responsibilities

Test

  • Mechanisms separating critical system components and functions
Official source-control relationships

Source NIST SP 800-53 controls

These controls are referenced by the official SP 800-172 Rev. 3 OSCAL record. Open the corresponding control pages for complete control text, enhancements, D3FEND mappings, and related learning.

Source controlSC-7(29)Open SP 800-53 control →
Source record

Authoritative sources

Bare Metal Cyber is an independent educational publisher and is not affiliated with or endorsed by NIST. The official publications, the responsible federal agency, and the governing contract or agreement determine applicability, tailoring, assessment depth, and required implementation.