Segregation of networks

What the control asks

Within its networks the organization is to keep groups of users, information services and information systems apart. Purpose: divide the network along security boundaries and control traffic between them according to business need. Guidance: consider managing large networks by splitting them into separate domains, each kept apart from the internet. Domains can be based on trust, criticality and sensitivity (for example public access, desktop and server domains, or low- and high-risk systems), on organizational units such as HR, finance or marketing, or on a mix, such as a server domain serving several units, and can be built with physically separate or logically separate networks. Each domain has a well-defined perimeter; where traffic between domains is allowed it passes through a gateway such as a firewall or filtering router. The criteria for dividing domains and the access permitted through gateways come from assessing each domain's security requirements against the access control policy (5.15), access needs and the value and classification of the information, taking into account the cost and performance impact of the gateway technology. Wireless networks need special handling because their perimeter is poorly defined: consider adjusting radio coverage, and in sensitive environments treat all wireless access as external, keeping it apart from internal networks until it has passed a gateway that applies the network controls (8.20). Guest wireless should be separated from staff wireless where staff use only controlled, policy-compliant devices, and guest WiFi should be at least as restricted as staff WiFi to discourage staff from using it. Other information: networks often extend beyond the organization through partnerships and shared facilities, raising the risk of unauthorized access to systems that need protecting from other network users.

Purpose in the standard’s terms, in our words: Divides the network along security boundaries and governs traffic across them by business need.

Attribute values in one line: Preventive; Confidentiality, Integrity, Availability; Protect; System and network security; Protection.

The five attribute values

These are the published values for this control, from ISO/IEC 27002:2022 clause 4.2 and Annex A Table A.1. Where your own value differs, record the difference and the reason in the same row rather than replacing the published value.

Evidence an auditor asks for

The attribute is a claim about how the control behaves. The evidence is what proves it operates. Keep both, and keep them together.

• Network segmentation design defining domains by trust, criticality, sensitivity or organizational unit, with the assessment that justified it
• Firewall or filtering router rules controlling traffic between domains, with rule review records
• Evidence that sensitive environments treat wireless as external and route it through a gateway
• Configuration showing guest wireless separated from staff networks with restrictions at least as strict
• Diagrams and controls for connections to partner networks
• Segmentation testing results confirming that disallowed traffic between domains is blocked

Where audits commonly find gaps

• A flat network lets any workstation reach servers and management interfaces
• Guest WiFi has fewer restrictions than staff WiFi, so staff use it to bypass controls
• Inter-domain rules have become overly permissive 'any-any' rules
• Partner network connections land inside internal networks without a gateway

What ISO/IEC 27028 changes for this control

It carries a single control type, preventive, which sets what the evidence has to prove. It contributes no detect function, so a control set built from this row alone would not show detection. Its earlier numbering (13.1.3) is kept in the register so an older audit file can still be traced.

The guidance expands how the five values are used; it does not change the control or add a requirement. A register that carries the values and the evidence can answer the attribute question for this control without a new document. A register that does not can add the columns at the next SoA version, with an owner and a review date.

Controls that share its values

• 8.18 Use of privileged utility programs
• 8.20 Networks security
• 8.21 Security of network services

Read the five attributes All 93 controls