Ethernet

What is Ethernet?

Ethernet is the world’s dominant networking technology, standardised as IEEE 802.3, which defines how devices on a local area network (LAN) exchange data in frames over copper cable or optical fibre. Ethernet covers the physical layer and the data link layer (layers 1 and 2) of the OSI model; protocols such as IP, TCP and UDP run on top of it. In offices Ethernet connects PCs and servers; in industrial automation it connects PLCs, HMIs, SCADA, Drives and IO modules.

🕰️ How did Ethernet come about?

Robert Metcalfe described Ethernet in an internal memo at Xerox PARC on 22 May 1973, and together with David Boggs he built the first network, running at roughly 3 Mbit/s. In 1980 DEC, Intel and Xerox published the 10 Mbit/s specification known as the DIX standard. The IEEE used it as the basis for IEEE 802.3, which was approved in 1983 for 10BASE5 over thick coaxial cable up to 500 metres.

Since then Ethernet has become ever faster while the frame format has stayed almost unchanged; 1.6 Tbit/s follows with IEEE 802.3dj. That is why a device from 1995 still works with a modern switch.

🧠 How does Ethernet work?

Every Ethernet device has a unique 48-bit MAC Address. Data is packed into a frame with a fixed structure:

A frame is therefore at least 64 and at most 1,518 bytes long (1,522 with a VLAN tag). Originally all devices shared one cable and CSMA/CD decided who could transmit. Modern networks use a switch: it learns which MAC address sits behind which port and forwards frames only there, in full duplex and without collisions. Managed switches add VLANs, QoS, SNMP management and mirror ports (SPAN).

For a step-by-step explanation of frames, MAC addresses and switching, see How Ethernet Works.

🔌 Which cables and connectors does Ethernet use?

• RJ45 with twisted pair — the standard in offices and control cabinets; Cat5e for 1 Gbit/s, Cat6A for 10 Gbit/s, up to 100 metres per channel
• M12 connectors — screw-locked and watertight for machines; D-coded (4-pin) for 100 Mbit/s, X-coded (8-pin) up to 10 Gbit/s
• Optical fibre — for distances from hundreds of metres to tens of kilometres, with galvanic isolation and immunity to electromagnetic interference, for example between buildings or around variable-frequency drives
• Single Pair Ethernet (10BASE-T1L) — 10 Mbit/s over a single pair up to 1,000 metres, including power; the basis of Ethernet-APL for process instruments, including in hazardous areas

With Power over Ethernet (PoE), the same cable also powers cameras, access points or sensors:

🏭 What is the difference between office Ethernet and Industrial Ethernet?

Industrial Ethernet uses the same IEEE 802.3 foundation, but places much tougher demands on hardware and timing:

Industrial protocols were developed for real-time control:

• ProfiNET (PROFIBUS & PROFINET International) — RT for standard I/O, IRT for motion control with cycle times down to 31.25 µs and jitter below 1 µs
• IP (ODVA) — carries the Common Industrial Protocol over standard TCP/UDP/IP
• EtherCAT (EtherCAT Technology Group) — a single frame passes through every device, which reads and writes its data on the fly
• Modbus TCP — simple and widespread, but neither deterministic nor secured by design; the Modbus/TCP Security variant with TLS (2018) is still rarely used
• TSN (Time-Sensitive Networking) — IEEE 802.1 extensions such as time synchronisation (802.1AS, based on the Precision Time Protocol) and scheduled traffic (802.1Qbv) that let real-time and IT traffic share one network; the IEC/IEEE 60802 profile for industrial automation was published in 2026

🔄 How does Ethernet compare with fieldbuses?

Classic fieldbuses such as Profibus and CAN are increasingly being replaced by Ethernet, but are still widely used in existing installations.

Because Ethernet and IT traffic share the same foundation, Ethernet enables OT convergence: O in the field talks to the PLC, and data flows from the machine to IT systems such as MES, Historian and Cloud applications. Architectures such as Converged Plantwide Ethernet describe how to approach this in a structured way.

🔐 How do you secure an Ethernet network in OT?

Ethernet itself has no authentication or encryption: in principle any connected device can send and receive frames. This makes Spoofing, ARP Spoofing, sniffing and DoS attacks relatively easy. Key security measures:

• VLANs and network segmentation — separate production, engineering and office, and guard the boundaries with firewalls, following the zones and conduits model of IEC 62443
• Port security — limit which MAC addresses are allowed per switch port and disable unused ports
• 802.1X and NAC — make devices authenticate before a port passes data, so that a rogue device gets no access
• Dynamic ARP Inspection — stops an attacker from impersonating a gateway or PLC
• Secured switch management — use SSH and HTTPS instead of Telnet and HTTP, and change default passwords
• Monitoring — SIEM, IDS and passive network monitoring via a SPAN port detect anomalous traffic
• Zero Trust and RBAC — give users and systems only the access they need

🧭 How do you design an industrial Ethernet network?

1. Inventory devices and protocols — which PLCs, drives and I/O use which protocol and which cycle time?
2. Divide the network into zones — use the Purdue Model and IEC 62443-3-2 to define zones and conduits.
3. Choose the physical layer — Cat6A or fibre for the backbone, M12 at the machine, fibre for distances over 100 metres or high interference.
4. Plan redundancy — an MRP ring for production lines, PRP for processes that cannot tolerate any interruption.
5. Select managed industrial switches — with VLANs, QoS, IGMP snooping for multicast traffic and security features.
6. Document everything and monitor — record ports and VLANs and analyse traffic with tools such as Wireshark.

In the Netherlands and the wider EU, many networks in the process industry, energy and drinking water fall under NIS2, which in the Netherlands has applied since 15 August 2026 through the Cyberbeveiligingswet (Cbw). The act requires appropriate network and information security measures; a well-segmented, documented Ethernet network is a basic prerequisite for compliance.

❓ Frequently asked questions

What is the difference between Ethernet and the internet?

Ethernet is a technology for local networks that transports frames between devices at layers 1 and 2. The internet is a worldwide network of networks that uses the IP protocol. Ethernet is often the local transport over which IP traffic travels to the internet.

How long can an Ethernet cable be?

An Ethernet link over twisted-pair copper cable can be at most 100 metres long, including patch cords. For longer distances you use optical fibre, which spans hundreds of metres to tens of kilometres. Single Pair Ethernet according to 10BASE-T1L reaches 1,000 metres at 10 Mbit/s.

Why is standard Ethernet not deterministic?

Standard Ethernet handles frames on a best-effort basis: when the network is busy, frames wait in switch buffers, so latency varies. Industrial protocols such as PROFINET IRT and EtherCAT, and the TSN standards from IEEE 802.1, solve this with priorities, time windows and clock synchronisation. This makes Ethernet suitable for motion control.

Is Industrial Ethernet a separate standard?

Industrial Ethernet is not a separate IEEE standard but the collective name for Ethernet with ruggedised hardware and industrial protocols. The physical layer remains IEEE 802.3, while protocols such as PROFINET, EtherNet/IP and EtherCAT are defined in IEC 61158 and IEC 61784.

What is Single Pair Ethernet?

Single Pair Ethernet is Ethernet over a single twisted pair instead of two or four. The 10BASE-T1L variant from IEEE 802.3cg (2019) reaches 10 Mbit/s over 1,000 metres and can power the device at the same time. Ethernet-APL uses this technology to bring Ethernet all the way to process instruments in hazardous areas.

How much power does Power over Ethernet deliver?

Power over Ethernet delivers up to 15.4 W per port under IEEE 802.3af, 30 W under 802.3at and up to 90 W under 802.3bt at the switch side. Because of cable losses, slightly less reaches the device: 71.3 W for 802.3bt Type 4. That is enough for cameras, access points and small industrial panels.

📌 In summary

Ethernet (IEEE 802.3) has been the standard for local networks since 1983 and is the backbone of both office and industrial networks, from 10 Mbit/s to 800 Gbit/s. In industry, Ethernet is reinforced with ruggedised hardware, real-time protocols such as PROFINET and EtherCAT, TSN and tight segmentation according to IEC 62443 for reliability and safety.