Industrial Ethernet and TCP/IP Basics: IP Addresses, Cabling and Wiring for Plant Networks

For two devices to talk over Ethernet, three things must be right: a physical link (cable and switch port), an IP address on the correct network, and a protocol both devices understand. Most connection problems on site come from one of those three.

Diagram: RJ45 T568B wire colours by pin, and an example plant network with a switch, PLC, data logger and server on one subnet

Tap or click the diagram to enlarge it.

TCP/IP

TCP/IP in Three Ideas

You do not need to be a network engineer, but these three ideas solve most problems.

IP address

Every device needs a unique address, for example 192.168.1.20 (an example of a private address). Two devices with the same address cause conflicts.

Subnet mask and gateway

Devices on the same subnet, for example mask 255.255.255.0, talk to each other directly. The gateway is the router address used to reach other networks.

Port number

A port identifies the service on a device. Modbus TCP normally uses 502, OPC UA commonly 4840, MQTT 1883 (8883 with TLS), and EtherNet/IP 44818.

Cabling

Cables and Connectors

The diagram above shows the T568B pinout and an example network.

1
Up to 100 m per runTwisted-pair Ethernet (Cat5e or better) is limited to 100 m per channel. For longer runs, use a switch, a media converter or fibre.
2
Wire both ends the same wayUse T568B at both ends (or T568A at both). A straight-through cable has identical wiring at both ends. Modern ports detect the pairing automatically, so crossover cables are rarely needed.
3
Pins 1, 2, 3 and 6 carry 10/100 Mbit/sFast Ethernet uses two pairs. Gigabit uses all four, so a damaged or two-pair cable may link at 100 Mbit/s only.
4
Use industrial parts in harsh areasShielded cable, locking or M12 connectors and industrial switches cope better with vibration, dust, moisture and electrical noise.
5
Separate from power cablesKeep Ethernet away from motor and inverter cables, and ground shielded cable as the manufacturer instructs.

Network Habits

Keeping the Network Reliable

Small habits prevent most late-night callouts.

1
Keep an address listRecord every device, its IP address and its purpose. Use fixed (static) addresses for devices that others must find.
2
Choose the right switchUnmanaged switches are simple. Managed switches add VLANs, monitoring and port control, which help on larger plants.
3
Separate plant and office networksPut control devices on their own network or VLAN, with a firewall between them and the office network.
4
Never expose Modbus TCP to the InternetModbus has no built-in authentication or encryption. Reach remote sites through a VPN or a secure gateway.
5
Label cables and portsA label on each cable and switch port lets the next engineer trace a fault in minutes.

IP addresses in this article are examples only. Use the addressing plan of your own site. Port numbers and cable limits are typical values; confirm them in the device and cable documentation.

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Industrial Ethernet and TCP/IP Basics: Addresses, Cabling and Wiring