Segregation of networks

What the requirement 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. In the ISMS the control is compared with the controls determined in risk treatment and recorded in the Statement of Applicability as included or excluded, with the reason and the implementation status.

What the guidance adds

Segregation limits the blast radius. The guidance asks for the zone, the boundary, the rule and the test.

What good looks like

Put the control in the Statement of Applicability with an owner, a status and the risk it treats. Then keep network segmentation design defining domains by trust, criticality, sensitivity or organizational unit, with the assessment that justified it and one live sample together.

Evidence an assessor asks for

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

The question an assessor will ask

An assessor will ask for network segmentation design defining domains by trust, criticality, sensitivity or organizational unit, with the assessment that justified it, then trace it to the system it protects. For A.8.22 the test is whether that record ties to a live sample, not whether the control is named in a policy.

Common 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

Before the next audit

Before the next audit, check one live sample for this gap: a flat network lets any workstation reach servers and management interfaces.