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Industrial network basics: VLANs, PoE and cable limits
How plant networks separate control, CCTV and access control with VLANs and a firewall, and the practical limits of copper, fibre and PoE.
Why separate networks
A plant network often carries very different traffic: PLCs and HMIs, security cameras, door controllers and office computers. Putting them in separate VLANs keeps a camera problem or a broadcast storm from disturbing the control system and makes it easier to control who can reach what.
Switches, trunks and the firewall
Devices in the same VLAN talk through the switches directly. Links between switches are configured as trunks that carry several VLANs. Traffic between VLANs goes through a router or firewall, which allows only the connections listed in its policy, for example letting the HMI view cameras while blocking cameras from reaching the PLC.
Cable limits
Twisted-pair copper Ethernet is designed for channels up to 100 m. Longer runs need fibre or an additional switch. Fibre reach depends on the fibre type and the transceiver, so use the value from the transceiver data sheet.
Power over Ethernet and bandwidth
PoE switches power cameras and other devices through the data cable, within a total power budget. If the budget is exceeded, some devices will not start. Every camera stream also adds load to every link on its way to the recorder, so uplinks need enough capacity, with spare.
Try it yourself
Open the Network tab, run connection tests between devices, cut the fibre link and lower the PoE budget to see the effect.
Open the PlantFlow 3D simulator →Safety: this guide explains principles for learning. Real installations must follow the applicable codes, the equipment manufacturer's instructions and the approval of a competent, qualified person. Isolate, lock off and prove dead before touching any circuit.