The zone-and-conduit model is an OT security design that groups assets with similar criticality, function, or trust level into zones, then allows communication only through defined conduits. A zone might contain PLCs, HMIs, or medical devices with similar operational needs; a conduit is the controlled path between zones, usually enforced by firewalls, allowlists, monitoring, and protocol restrictions.
This model matters because OT environments must balance security with uptime and safety. Instead of assuming a VLAN or diagram equals isolation, defenders verify which systems can actually talk, on which ports, and for what purpose. In attacks, weak or flattened segmentation lets malware or an intruder move laterally from enterprise IT into control networks. In defense, zones and conduits reduce blast radius, make exceptions visible, and support continuous validation of east-west traffic. The model is widely used because it turns segmentation from a label into an enforceable security control.