implementing-network-segmentation-for-ot

This skill covers implementing network segmentation in Operational Technology environments using VLANs, industrial firewalls, data diodes, and software-defined networking. It addresses the Purdue Model-based segmentation strategy, migration from flat networks to segmented architectures without disrupting operations, configuring OT-aware firewalls with industrial protocol deep packet inspection, and validating segmentation effectiveness through traffic analysis.

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name	implementing-network-segmentation-for-ot
description	This skill covers implementing network segmentation in Operational Technology environments using VLANs, industrial firewalls, data diodes, and software-defined networking. It addresses the Purdue Model-based segmentation strategy, migration from flat networks to segmented architectures without disrupting operations, configuring OT-aware firewalls with industrial protocol deep packet inspection, and validating segmentation effectiveness through traffic analysis.
domain	cybersecurity
subdomain	ot-ics-security
tags	["ot-security","ics","scada","industrial-control","iec62443","network-segmentation","vlan"]
version	1.0.0
author	mahipal
license	Apache-2.0

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 implementing-zero-trust-in-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: 
    devices:  = field(default_factory=)



 :
    rule_id: 
    source_zone: 
    source_ip: 
    dest_zone: 
    dest_ip: 
    protocol: 
    port: 
    action: 
    dpi_profile:  = 
    comment:  = 


 :
    

     ():
         (baseline_file)  f:
            .baseline = json.load(f)
        .vlans = []
        .rules = []
        .rule_counter = 

     ():
        
        .vlans = [
            VLANDesign(, , ,
                       , ,
                       ),
            VLANDesign(, , ,
                       , ,
                       ),
            VLANDesign(, , ,
                       , ,
                       ),
            VLANDesign(, , ,
                       , ,
                       ),
            VLANDesign(, , ,
                       , ,
                       ),
            VLANDesign(, , ,
                       , ,
                       ),
            VLANDesign(, , ,
                       , ,
                       ),
        ]
         .vlans

     ():
        
        .rules = []

        
        zone_pairs = [
            (, ),
            (, ),
            (, ),
            (, ),
        ]

        
         flow  .baseline.get(, []):
            .rules.append(FirewallRule(
                rule_id=.rule_counter,
                source_zone=flow[],
                source_ip=flow[],
                dest_zone=flow[],
                dest_ip=flow[],
                protocol=flow.get(, ),
                port=flow.get(, ),
                action=,
                dpi_profile=._get_dpi_profile(flow.get(, )),
                comment=,
            ))
            .rule_counter += 

        
         src_zone, dst_zone  zone_pairs:
            .rules.append(FirewallRule(
                rule_id=.rule_counter,
                source_zone=src_zone,
                source_ip=,
                dest_zone=dst_zone,
                dest_ip=,
                protocol=,
                port=,
                action=,
                comment=,
            ))
            .rule_counter += 

         .rules

     ():
        
        dpi_profiles = {
            : ,
            : ,
            : ,
            : ,
            : ,
        }
         dpi_profiles.get(port, )

     ():
        
        plan = {
            : {
                : ,
                : ,
                : [
                    ,
                    ,
                    ,
                    ,
                    ,
                    ,
                ],
                : ,
            },
            : {
                : ,
                : ,
                : [
                    ,
                    ,
                    ,
                    ,
                    ,
                    ,
                ],
                : ,
            },
            : {
                : ,
                : ,
                : [
                    ,
                    ,
                    ,
                    ,
                    ,
                ],
                : ,
            },
            : {
                : ,
                : ,
                : [
                    ,
                    ,
                    ,
                    ,
                    ,
                ],
                : ,
            },
        }
         plan

     ():
        
        design = {
            : [asdict(v)  v  .vlans],
            : [asdict(r)  r  .rules],
            : .generate_migration_plan(),
        }

         (output_file, )  f:
            json.dump(design, f, indent=)

        ()
        ()
        ()

         design


 __name__ == :
     (sys.argv) < :
        ()
        sys.exit()

    designer = SegmentationDesigner(sys.argv[])
    designer.design_vlans()
    designer.generate_firewall_rules_from_baseline()

    output = sys.argv[]  (sys.argv) >   
    designer.export_design(output)

# 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

#!/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()

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

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]