EtherNet/IP Protocol

What is EtherNet/IP?

EtherNet/IP is an industrial Ethernet protocol managed by ODVA (Open DeviceNet Vendors Association). Despite its name, the "IP" in EtherNet/IP stands for Industrial Protocol, not Internet Protocol. It was introduced in 2001 and has since become the dominant industrial Ethernet standard in North America, largely due to its adoption by Rockwell Automation (Allen-Bradley) across their ControlLogix and CompactLogix PLC platforms.

At its core, EtherNet/IP uses CIP (Common Industrial Protocol) as its application layer. CIP is a media-independent protocol that also runs over DeviceNet (CAN-based) and ControlNet (time-critical), giving engineers a unified object model regardless of the physical network. EtherNet/IP simply wraps CIP messages inside standard TCP/IP and UDP/IP packets.

One of the key advantages of EtherNet/IP is that it runs on standard, unmodified Ethernet hardware. Unlike some competing protocols that require specialized network interface cards or switches, EtherNet/IP works with off-the-shelf managed switches, standard Cat5e/Cat6 cabling, and commodity network infrastructure. This makes deployment straightforward and reduces hardware costs. EtherNet/IP is defined as part of IEC 61158, the international standard for industrial communication networks.

CIP: The Common Industrial Protocol

CIP is the application layer that gives EtherNet/IP its intelligence. Rather than defining a simple register-based data model, CIP uses an object-oriented architecture where every device, every function, and every data point is represented as an object with attributes and services. This design makes CIP highly extensible and allows devices from different manufacturers to interoperate using a shared vocabulary.

The CIP object model is organized into three categories. Required objects must be present in every CIP device and include the Identity Object (device name, vendor, serial number), the Message Router (directs incoming requests to the correct object), and the Connection Manager (establishes and manages connections). Application objects are specific to device types, such as the Assembly Object, which maps collections of IO data into a single addressable unit. Vendor-specific objects allow manufacturers to expose proprietary features while remaining CIP compliant.

Because CIP is shared across EtherNet/IP, DeviceNet, and ControlNet, a controller can use the same object model and service codes regardless of which network the target device is on. A read attribute service is the same whether the device is connected over Ethernet or a legacy DeviceNet segment, simplifying multi-network architectures.

EtherNet/IP Network Topology

EtherNet/IP networks use two primary device roles. A Scanner (typically a PLC or controller) initiates connections to target devices and schedules IO data exchanges. An Adapter (an IO module, drive, or sensor) responds to the scanner and produces or consumes data as configured. A single scanner can manage hundreds of adapter connections simultaneously.

EtherNet/IP supports a producer-consumer communication model, which is a significant departure from traditional polling. Instead of the controller requesting data from each device one at a time, a producing device can multicast its data onto the network, and any number of consuming devices can receive it directly. This enables device-to-device communication without every message routing through the scanner, reducing latency and network traffic for peer-to-peer applications.

Because EtherNet/IP runs on standard Ethernet, it supports star, linear (daisy-chain), and ring topologies using standard managed switches. Ring topologies with Device Level Ring (DLR) provide network redundancy with sub-second failover. For larger networks, standard spanning tree protocols or managed redundancy schemes can be used.

Implicit vs Explicit Messaging

EtherNet/IP defines two fundamentally different messaging modes, each optimized for a specific type of communication. Understanding the distinction between implicit and explicit messaging is essential for designing efficient EtherNet/IP networks.

Implicit messaging (also called IO messaging) is designed for real-time, cyclic exchange of process data. It uses UDP for transport, either unicast or multicast, and carries minimal protocol overhead. The scanner establishes a CIP connection with a defined RPI (Requested Packet Interval), and the adapter then produces data at that interval automatically. This is how IO points, drive feedback, and sensor readings are communicated in real time.

Explicit messaging is used for non-time-critical operations such as device configuration, parameter reads and writes, diagnostics, and firmware updates. It uses TCP for reliable delivery and follows a request-response pattern. Each message carries the full CIP path to the target object and attribute, making it flexible but less efficient than implicit messaging.

RPI: Requested Packet Interval

The Requested Packet Interval (RPI) defines the cycle time for implicit messaging connections. When a scanner opens a connection to an adapter, it specifies an RPI value that tells the adapter how often to produce its IO data. The RPI is configured per connection, so a single scanner can communicate with high-speed drives at 2 ms while polling temperature sensors at 100 ms.

Typical RPI values depend on the application. High-performance drives and motion axes typically require 1 to 10 ms. Standard discrete IO (digital inputs, outputs, valve manifolds) works well at 10 to 100 ms. Diagnostic and monitoring data can run at 100 ms or longer since it does not need fast updates. Choosing an RPI that is faster than necessary wastes network bandwidth and increases CPU load on both the scanner and adapter.

The scanner is responsible for scheduling all its implicit connections so that traffic is distributed evenly across each scan cycle. If too many connections are configured with aggressive RPI values, the scanner may not be able to service them all within the requested interval, leading to connection timeouts. Network planning tools from controller vendors help engineers calculate bandwidth utilization and verify that the selected RPI values are achievable for a given network topology.

EtherNet/IP vs Other Industrial Protocols

EtherNet/IP competes primarily with PROFINET and Modbus TCP in the industrial Ethernet space. Each protocol has distinct strengths, regional preferences, and architectural trade-offs. The following table summarizes the key differences.

EtherNet/IP's reliance on standard TCP/UDP means it works with any managed switch, while PROFINET RT uses Layer 2 prioritization and PROFINET IRT requires specialized hardware for sub-millisecond determinism. Modbus TCP is far simpler but lacks a real-time IO mechanism, making it better suited for monitoring and supervisory applications rather than closed-loop control.

Common Use Cases

• Discrete manufacturing: automotive assembly lines, packaging machines, and food and beverage production lines where EtherNet/IP connects PLCs, IO modules, and HMIs across the plant.
• Motion control: CIP Motion over EtherNet/IP enables synchronized multi-axis servo control with deterministic timing, commonly used in CNC machines and robotic cells.
• Process automation: chemical, pharmaceutical, and oil and gas facilities use EtherNet/IP for both control and safety layers, with CIP Safety providing SIL 3 safety-rated communication over the same network.
• Material handling: conveyor systems, sortation equipment, and warehouse automation rely on EtherNet/IP for coordinating drives, sensors, and barcode readers.
• Machine tool control: CNC lathes and milling machines use EtherNet/IP for both IO control and real-time drive feedback.
• IT/OT convergence: because EtherNet/IP runs on standard IP infrastructure, it integrates naturally with enterprise networks, enabling data collection for analytics, MES systems, and cloud platforms without protocol translation gateways.

Frequently Asked Questions About EtherNet/IP

What does the IP in EtherNet/IP stand for?

The IP stands for Industrial Protocol. This is a common source of confusion since IP also stands for Internet Protocol in the TCP/IP stack. ODVA chose the name to emphasize that EtherNet/IP is purpose-built for industrial automation, even though it does run over standard Internet Protocol networking.

Does EtherNet/IP work with standard Ethernet switches?

Yes. EtherNet/IP is designed to run on standard, unmodified Ethernet infrastructure. Any managed switch that supports IGMP snooping (for multicast management) and QoS (for traffic prioritization) will work. No proprietary or specialized switch hardware is required, though industrial-grade switches are recommended for harsh environments.

What is CIP and why does it matter?

CIP (Common Industrial Protocol) is the application layer that runs on top of EtherNet/IP. It provides an object-oriented data model with standardized device profiles, services, and connection management. CIP matters because it gives EtherNet/IP rich device interoperability: a drive from one vendor and an IO module from another can communicate using the same object definitions and service codes, reducing integration effort.

What is the difference between EtherNet/IP and PROFINET?

Both are industrial Ethernet protocols, but they differ in architecture and market focus. EtherNet/IP uses standard TCP/UDP for all communication, including real-time IO (via UDP implicit messaging). PROFINET uses a dedicated Layer 2 protocol for real-time data, bypassing TCP/IP entirely for faster cycle times. EtherNet/IP dominates in North America (Rockwell ecosystem), while PROFINET dominates in Europe (Siemens ecosystem).

Can EtherNet/IP devices communicate directly with each other?

Yes. The producer-consumer model allows any producing device to multicast its data so that multiple consumers can receive it without the data passing through a scanner. This is useful for device-to-device interlocking, drive-to-drive coordination, and peer IO sharing. The scanner still configures and monitors these connections, but the data flows directly between devices.

What is CIP Safety?

CIP Safety is a safety-rated extension of CIP that enables safety-critical communication over EtherNet/IP (and DeviceNet / ControlNet). It is certified to SIL 3 per IEC 61508 and Performance Level e per ISO 13849. CIP Safety uses additional data integrity checks, time stamps, and unique connection IDs to ensure that safety messages are delivered correctly, even over a shared network carrying standard traffic.

Related Protocols

• PROFINET: Siemens-backed industrial Ethernet protocol with Layer 2 real-time capabilities.
• Modbus TCP: simple register-based industrial protocol running over TCP/IP on port 502.
• Modbus RTU: the original serial Modbus protocol for RS-485 and RS-232 networks.
• TCP: the reliable transport layer protocol that carries EtherNet/IP explicit messaging.