Network segmentation divides your infrastructure into isolated zones, each with its own access controls and security policies. This architectural approach limits the damage an attacker can inflict after gaining initial access by preventing unrestricted movement between systems. Without segmentation, a single compromised workstation can reach every server, database, and application in the environment.
Flat networks, where all devices share the same network segment and can communicate freely, remain surprisingly common in organisations of all sizes. The convenience of universal connectivity comes at a severe security cost. Attackers who breach the perimeter of a flat network face no additional barriers. They can scan for vulnerable systems, harvest credentials, and access sensitive data across the entire infrastructure.
Effective segmentation begins with understanding your data flows. Which systems need to communicate with which others, and over which protocols? Mapping these legitimate communication patterns reveals the minimum connectivity required for business operations. Everything beyond that minimum represents unnecessary risk that segmentation can eliminate.
Critical assets deserve their own segments with strict access controls. Payment processing systems, databases containing personal information, intellectual property repositories, and administrative management platforms should each reside in protected zones accessible only to authorised users and systems. This isolation ensures that compromising a less critical system does not provide a path to your most valuable assets.
Micro-segmentation takes the concept further by applying controls at the individual workload level. Rather than grouping systems into broad network zones, micro-segmentation policies define precisely which processes on which systems can communicate with which other processes. This granularity limits lateral movement to the point where compromising a single system provides almost no access to anything else.
Expert Commentary
William Fieldhouse | Director of Aardwolf Security Ltd
“Flat networks are an attacker’s playground. Once they gain access to one system, every other system is reachable without additional barriers. Proper segmentation forces attackers to break through multiple boundaries, giving defenders time to detect and respond. It is one of the most effective controls we recommend, and one of the most commonly absent.”
Regular internal network penetration testing validates that segmentation controls actually prevent the lateral movement they are designed to stop. Professional testers attempt to cross segment boundaries, bypass firewall rules, and reach protected assets from various starting positions within the network. These tests reveal gaps between intended segmentation design and actual enforcement.
Monitoring traffic between segments detects attempts to cross security boundaries. Legitimate cross-segment traffic follows predictable patterns. Anomalous connections, port scans targeting other segments, and unexpected protocol usage all indicate potential compromise or misconfiguration that requires investigation.
Cloud environments require segmentation strategies adapted to virtual networking constructs. Virtual private clouds, security groups, and network access control lists provide the tools for cloud segmentation, but their configuration differs significantly from traditional network firewalls. Organisations migrating to the cloud must redesign their segmentation rather than simply replicating on-premises architecture.
Complementary external network penetration testing examines whether segmentation extends to your internet-facing perimeter. External testers determine whether compromising a public-facing service provides any path to internal segments that should be unreachable from outside the organisation.
Segmentation is not a one-time project. Network architectures evolve as businesses grow, applications change, and new technologies arrive. Regular review of segmentation policies, combined with ongoing testing, ensures that isolation controls keep pace with infrastructure changes. The investment in segmentation pays its greatest dividends when a breach occurs, because the difference between a contained incident and a catastrophic compromise often comes down to whether effective segmentation was in place.
