CYBRARY Unit 1 — Zones, conduits and levels
12 minLevels, zones and the plant floor range
The Purdue levels bottom to top, and the five zones the Forge plant range is actually cut into.
By the end you can
- Place the Purdue levels in order from the physical process up to business IT.
- Name the five zones on the range and what lives in each.
- Recognize when two nominal zones are really one zone.
The levels, bottom to top
Control engineers stack a plant into reference levels. Level 0 is the metal that moves; level 4 is the corporate network with mail in it. The DMZ at 3.5 is the brokered gap between the two worlds.
| Level | What lives there |
|---|---|
| L0 — Process | Sensors, valves and drives — the physical plant itself. |
| L1 — Basic control | PLCs and safety controllers running the logic. |
| L2 — Supervisory | HMIs, SCADA servers and engineering workstations. |
| L3 — Site operations | Historians, batch records and plant applications. |
| L3.5 — Industrial DMZ | The brokered boundary: jump hosts, replicated historian, patch staging. |
| L4 — Enterprise | Business IT — mail, ERP, the corporate directory. |
The zones on the range
The plant floor range is built to those levels. These are the zones you can open, with the target security level each is held to.
| Zone | What it holds |
|---|---|
| Safety instrumented system (L0/L1, target SL3) | Safety controller and its interlocks, on its own conduit and never routed. |
| Line control zone (L1/L2, target SL3) | PLC-201 and PLC-204, HMI-110, the serial gateway GW-260. |
| Site operations zone (L3, target SL2) | HIST-320 process historian, batch records, the engineering workstation. |
| Industrial DMZ (L3.5, target SL2) | Replicated historian, jump host, patch staging — the only way in from business IT. |
| Enterprise zone (L4, target SL1) | Corporate IT. Treated as untrusted by everything below it. |
The test for a real zone
If you cannot name the conduit between two groups of equipment, they are not two zones. They are one zone with a longer cable, and the risk of the weaker half is the risk of both.