undermybelt Implementing Network Segmentation for OT
When to Use
- When an OT security assessment reveals a flat network with no segmentation between Purdue levels
- When implementing IEC 62443 zone/conduit architecture after completing risk assessment (IEC 62443-3-2)
- When separating IT and OT networks as part of an IT/OT convergence security initiative
- When deploying a DMZ between corporate IT and OT to protect industrial systems from IT-originating threats
- When segmenting safety instrumented systems (SIS) from basic process control systems (BPCS)
Do not use for IT-only microsegmentation without OT components (see zero-trust-cloud), or for initial zone design without prior traffic analysis (see performing-ot-network-security-assessment first).
Prerequisites
- Complete traffic baseline from passive monitoring (minimum 2-4 weeks of capture data)
- Asset inventory with Purdue level classifications for all OT devices
- Industrial-grade network switches with VLAN support and port security
- OT-aware firewalls (Cisco ISA-3000, Fortinet FortiGate Rugged, Palo Alto with OT Security)
- Maintenance window schedule for network changes
- Rollback plan approved by operations management
Workflow
Step 1: Design Segmentation Architecture Based on Traffic Baseline
Use the traffic baseline to design VLAN and firewall architecture that preserves all legitimate communication paths while isolating zones.
#!/usr/bin/env python3
"""OT Network Segmentation Design Tool.
Analyzes traffic baseline data and generates a segmentation design
with VLAN assignments, firewall rules, and migration plan.
"""
import json
import sys
from collections import defaultdict
from dataclasses import dataclass, field, asdict
from ipaddress import ip_address, ip_network
@dataclass
class VLANDesign:
vlan_id: int
name: str
purdue_level: str
subnet: str
gateway: str
description: str
devices: list = field(default_factory=list)
@dataclass
class FirewallRule:
rule_id: int
source_zone: str
source_ip: str
dest_zone: str
dest_ip: str
protocol: str
port: int
action: str
dpi_profile: str = ""
comment: str = ""
class SegmentationDesigner:
"""Generates segmentation design from traffic baseline."""
def __init__(self, baseline_file):
with open(baseline_file) as f:
self.baseline = json.load(f)
self.vlans = []
self.rules = []
self.rule_counter = 1
def design_vlans(self):
"""Create VLAN design based on Purdue levels."""
self.vlans = [
VLANDesign(10, "SIS-SAFETY", "Level 1 (Safety)",
"10.10.10.0/24", "10.10.10.1",
"Safety Instrumented Systems - air-gapped or hardware-isolated"),
VLANDesign(20, "BPCS-FIELD", "Level 0-1 (Field/Control)",
"10.10.20.0/24", "10.10.20.1",
"PLCs, RTUs, I/O modules, field instruments"),
VLANDesign(30, "BPCS-SUPERVISORY", "Level 2 (Supervisory)",
"10.10.30.0/24", "10.10.30.1",
"HMIs, engineering workstations, local historian"),
VLANDesign(40, "SITE-OPS", "Level 3 (Operations)",
"10.10.40.0/24", "10.10.40.1",
"Site historian, OPC server, MES, alarm management"),
VLANDesign(50, "OT-DMZ", "Level 3.5 (DMZ)",
"172.16.50.0/24", "172.16.50.1",
"Data diode, historian mirror, jump server, patch server"),
VLANDesign(60, "ENTERPRISE", "Level 4 (Enterprise)",
"10.0.60.0/24", "10.0.60.1",
"Enterprise IT systems accessing OT data"),
VLANDesign(999, "QUARANTINE", "Quarantine",
"10.10.99.0/24", "10.10.99.1",
"Quarantine VLAN for unauthorized or untrusted devices"),
]
return self.vlans
def generate_firewall_rules_from_baseline(self):
"""Generate firewall rules based on observed legitimate traffic."""
self.rules = []
# Default deny rules for each zone boundary
zone_pairs = [
("Level 2", "Level 0-1"),
("Level 3", "Level 2"),
("Level 3.5", "Level 3"),
("Level 4", "Level 3.5"),
]
# Generate allow rules from baseline observed traffic
for flow in self.baseline.get("cross_zone_flows", []):
self.rules.append(FirewallRule(
rule_id=self.rule_counter,
source_zone=flow["src_level"],
source_ip=flow["src"],
dest_zone=flow["dst_level"],
dest_ip=flow["dst"],
protocol=flow.get("protocol", "TCP"),
port=flow.get("port", 0),
action="ALLOW",
dpi_profile=self._get_dpi_profile(flow.get("port", 0)),
comment=f"Baseline observed: {flow['src']} -> {flow['dst']}",
))
self.rule_counter += 1
# Add default deny rules at the end of each zone ACL
for src_zone, dst_zone in zone_pairs:
self.rules.append(FirewallRule(
rule_id=self.rule_counter,
source_zone=src_zone,
source_ip="any",
dest_zone=dst_zone,
dest_ip="any",
protocol="any",
port=0,
action="DENY",
comment=f"Default deny: {src_zone} -> {dst_zone}",
))
self.rule_counter += 1
return self.rules
def _get_dpi_profile(self, port):
"""Return the appropriate DPI inspection profile for an OT protocol port."""
dpi_profiles = {
502: "modbus-inspect (allow read FC only from L3)",
44818: "enip-inspect",
4840: "opcua-inspect (require SignAndEncrypt)",
102: "s7comm-inspect",
20000: "dnp3-inspect",
}
return dpi_profiles.get(port, "none")
def generate_migration_plan(self):
"""Generate phased migration plan for network segmentation."""
plan = {
"phase_1": {
"name": "DMZ Implementation (Week 1-2)",
"description": "Deploy DMZ between enterprise and OT networks",
"steps": [
"Deploy DMZ firewall pair (inside and outside)",
"Migrate historian mirror to DMZ",
"Configure jump server in DMZ with MFA",
"Install data diode for unidirectional historian replication",
"Route enterprise-to-OT traffic through DMZ",
"Verify enterprise access to historian data via DMZ",
],
"rollback": "Remove DMZ firewall rules, restore direct routing",
},
"phase_2": {
"name": "L3/L2 Segmentation (Week 3-4)",
"description": "Separate operations (L3) from control (L2) zones",
"steps": [
"Create VLAN 30 and VLAN 40 on OT switches",
"Deploy industrial firewall between L2 and L3",
"Configure firewall in monitor mode (log only, no blocking)",
"Analyze logs for 1 week to validate rule completeness",
"Switch to enforcement mode during maintenance window",
"Validate all HMI-to-PLC and historian-to-PLC communications",
],
"rollback": "Revert VLAN assignments, set firewall to permit-any",
},
"phase_3": {
"name": "Field Device Isolation (Week 5-6)",
"description": "Isolate Level 0-1 field devices from Level 2 supervisory",
"steps": [
"Create VLAN 20 for PLCs and field instruments",
"Configure port security with MAC binding on PLC ports",
"Apply Modbus function code filtering (block writes from L3)",
"Test all control loops during maintenance window",
"Verify alarm propagation from field to HMI",
],
"rollback": "Merge VLAN 20 back into VLAN 30",
},
"phase_4": {
"name": "SIS Isolation (Week 7-8)",
"description": "Fully isolate Safety Instrumented Systems",
"steps": [
"Verify SIS is on dedicated VLAN 10 or air-gapped",
"Remove any network path between SIS and BPCS",
"Implement dedicated engineering workstation for SIS",
"Apply USB and removable media controls on SIS EWS",
"Test SIS functionality in isolation",
],
"rollback": "N/A - SIS isolation should not be reversed",
},
}
return plan
def export_design(self, output_file):
"""Export complete segmentation design."""
design = {
"vlans": [asdict(v) for v in self.vlans],
"firewall_rules": [asdict(r) for r in self.rules],
"migration_plan": self.generate_migration_plan(),
}
with open(output_file, "w") as f:
json.dump(design, f, indent=2)
print(f"[*] Segmentation design exported to: {output_file}")
print(f" VLANs: {len(self.vlans)}")
print(f" Firewall Rules: {len(self.rules)}")
return design
if __name__ == "__main__":
if len(sys.argv) < 2:
print("Usage: python segmentation_designer.py <baseline.json> [output.json]")
sys.exit(1)
designer = SegmentationDesigner(sys.argv[1])
designer.design_vlans()
designer.generate_firewall_rules_from_baseline()
output = sys.argv[2] if len(sys.argv) > 2 else "segmentation_design.json"
designer.export_design(output)
Step 2: Configure Industrial Switch VLANs
Apply VLAN configuration to industrial Ethernet switches with port security and unused port hardening.
# Cisco Industrial Ethernet 4000/5000 Series Configuration
# Create VLANs aligned with Purdue levels
vlan 10
name SIS-SAFETY-L1
vlan 20
name BPCS-FIELD-L01
vlan 30
name BPCS-SUPERVISORY-L2
vlan 40
name SITE-OPS-L3
vlan 50
name OT-DMZ-L35
vlan 999
name QUARANTINE
# PLC access ports with port security
interface range GigabitEthernet1/0/1-12
description PLC Connections
switchport mode access
switchport access vlan 20
switchport port-security
switchport port-security maximum 1
switchport port-security mac-address sticky
switchport port-security violation shutdown
storm-control broadcast level 10
storm-control multicast level 10
spanning-tree portfast
spanning-tree bpduguard enable
no cdp enable
no lldp transmit
no lldp receive
# HMI access ports
interface range GigabitEthernet1/0/13-18
description HMI Stations
switchport mode access
switchport access vlan 30
switchport port-security
switchport port-security maximum 1
switchport port-security mac-address sticky
switchport port-security violation restrict
spanning-tree portfast
# Trunk to zone firewall
interface TenGigabitEthernet1/0/1
description Trunk to OT Zone Firewall
switchport mode trunk
switchport trunk allowed vlan 20,30,40,50
switchport trunk native vlan 999
switchport nonegotiate
# Disable and quarantine all unused ports
interface range GigabitEthernet1/0/19-48
description UNUSED - Shutdown
switchport mode access
switchport access vlan 999
shutdown
Step 3: Validate Segmentation Effectiveness
After implementation, validate that segmentation correctly blocks unauthorized cross-zone traffic while permitting all legitimate operations.
#!/usr/bin/env python3
"""OT Network Segmentation Validator.
Runs automated tests to verify zone isolation, firewall rules,
and protocol enforcement after segmentation deployment.
"""
import json
import socket
import subprocess
import sys
import time
from dataclasses import dataclass, asdict
@dataclass
class ValidationTest:
test_id: str
description: str
source_zone: str
target_ip: str
target_port: int
expected_result: str # "blocked" or "allowed"
actual_result: str = ""
status: str = "" # PASS or FAIL
class SegmentationValidator:
"""Validates OT network segmentation implementation."""
def __init__(self):
self.tests = []
self.results = []
def add_test(self, test):
self.tests.append(test)
def run_connectivity_test(self, target_ip, target_port, timeout=3):
"""Test TCP connectivity to target."""
try:
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.settimeout(timeout)
result = sock.connect_ex((target_ip, target_port))
sock.close()
return "reachable" if result == 0 else "blocked"
except (socket.timeout, ConnectionRefusedError):
return "blocked"
except Exception:
return "error"
def run_all_tests(self):
"""Execute all segmentation validation tests."""
print("=" * 60)
print("OT SEGMENTATION VALIDATION")
print("=" * 60)
passed = 0
failed = 0
for test in self.tests:
actual = self.run_connectivity_test(test.target_ip, test.target_port)
test.actual_result = actual
if actual == test.expected_result:
test.status = "PASS"
passed += 1
else:
test.status = "FAIL"
failed += 1
icon = "[+]" if test.status == "PASS" else "[-]"
print(f" {icon} {test.test_id}: {test.description}")
print(f" Target: {test.target_ip}:{test.target_port}")
print(f" Expected: {test.expected_result} | Actual: {actual} -> {test.status}")
print(f"\n Results: {passed} passed, {failed} failed out of {len(self.tests)} tests")
return {"passed": passed, "failed": failed, "total": len(self.tests)}
if __name__ == "__main__":
validator = SegmentationValidator()
# Tests from Enterprise zone (Level 4) - should be blocked from OT
validator.add_test(ValidationTest(
"SEG-001", "Enterprise cannot reach PLCs via Modbus",
"Level 4", "10.10.20.10", 502, "blocked"))
validator.add_test(ValidationTest(
"SEG-002", "Enterprise cannot reach PLCs via EtherNet/IP",
"Level 4", "10.10.20.10", 44818, "blocked"))
validator.add_test(ValidationTest(
"SEG-003", "Enterprise can reach DMZ jump server",
"Level 4", "172.16.50.10", 3389, "allowed"))
validator.add_test(ValidationTest(
"SEG-004", "Enterprise can reach DMZ historian mirror",
"Level 4", "172.16.50.20", 443, "allowed"))
# Tests from Operations zone (Level 3) - limited access to control
validator.add_test(ValidationTest(
"SEG-005", "Operations can read from PLCs via Modbus",
"Level 3", "10.10.20.10", 502, "allowed"))
validator.add_test(ValidationTest(
"SEG-006", "Operations cannot reach SIS controllers",
"Level 3", "10.10.10.10", 1502, "blocked"))
validator.run_all_tests()
Key Concepts
| Term | Definition |
|---|---|
| VLAN | Virtual Local Area Network - Layer 2 broadcast domain isolation used to separate OT zones on shared switch infrastructure |
| Industrial Firewall | Firewall with deep packet inspection capabilities for industrial protocols (Modbus, DNP3, EtherNet/IP, OPC UA) |
| Data Diode | Hardware-enforced unidirectional gateway that physically prevents reverse data flow, used between OT operations and DMZ |
| Port Security | Switch feature that limits the number of MAC addresses on a port and locks assignments, preventing unauthorized device connections |
| Trunk Port | Switch port carrying multiple VLANs using 802.1Q tagging, used to connect switches and firewalls across zone boundaries |
| DMZ | Demilitarized Zone between enterprise IT and OT - a buffer zone where all cross-domain traffic terminates and is inspected |
Tools & Systems
- Cisco ISA-3000: Industrial security appliance with Modbus, DNP3, and EtherNet/IP deep packet inspection for OT zone firewalls
- Fortinet FortiGate Rugged Series: Ruggedized NGFW with OT protocol support and industrial environment certifications
- Waterfall Security Unidirectional Gateway: Hardware data diode for enforcing one-way data flow from OT to IT
- Cisco Industrial Ethernet Switches: Managed switches with VLAN, port security, and industrial protocol support
Output Format
OT Network Segmentation Report
================================
Implementation Date: YYYY-MM-DD
VLAN ARCHITECTURE:
VLAN [ID] - [Name] ([Purdue Level])
Subnet: [subnet/mask]
Devices: [count]
FIREWALL RULES:
[Zone A] -> [Zone B]: [allow/deny count]
VALIDATION RESULTS:
Tests Passed: [N]/[Total]
Critical Failures: [N]