Network segmentation
Dividing a network into isolated zones so that devices in one zone cannot reach systems in another without passing through a controlled boundary.
Segmentation limits what a compromised device can touch. In a home, smart plugs and cameras go on a separate Wi-Fi network from laptops; in a plant, the control network is separated from the office network by firewalls, and IEC 62443 formalizes this as zones and conduits with defined security levels. An edge gateway often sits on the boundary, exposing only the data flows that are needed.
Segmentation is cheap compared with hardening every device, which is why it is the first recommendation for fleets of legacy equipment that cannot be updated. It is not a substitute for device security, since attackers who get inside a zone can still move within it, but combined with per-device identity it approaches a zero trust posture.
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