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EtherNet/IP protocol fundamentals

EtherNet/IP, short for Ethernet Industrial Protocol, is an open industrial network protocol that adapts the CIP (Common Industrial Protocol) to the physical and data link layers of standard Ethernet, enabling real-time communication and device configuration in automation environments with a typical transfer rate of 100 Mbit/s.

Protocol Architecture

Section titled “Protocol Architecture”

EtherNet/IP distributes its communication services in a hybrid TCP/IP model that assigns different functions to the transport layer: critical I/O traffic uses UDP to minimize latency through implicit messaging, while configuration and diagnostic tasks travel over TCP with explicit messaging. The protocol implements CIP in the upper layers (session, presentation, and application of the OSI model), encapsulating automation data using an object-oriented model common to other ODVA networks such as DeviceNet and ControlNet.

OSI Layer Protocol Implemented Main Function
Application (7) CIP Device profiles and specific objects
Presentation (6) CIP Data format and type translation
Session (5) CIP Connection establishment and management
Transport (4) TCP / UDP Flow control (TCP) or real-time priority (UDP)
Network (3) IP Routing and logical addressing
Data Link (2) Ethernet Media Access Control (MAC) and frames
Physical (1) Ethernet (IEEE 802.3) Electrical/optical signaling and cabling

Implicit and Explicit Messaging

Section titled “Implicit and Explicit Messaging”

The protocol defines two data exchange mechanisms differentiated by their purpose and transport resource. Implicit messaging transfers real-time input/output data with low overhead via UDP packets, using predefined connection identifiers and without needing to interpret commands. Explicit messaging uses TCP for requests and responses with a request/service structure, transporting commands such as reading and writing parameters, configuration, and program uploads, where each message includes a service code, class, instance, and CIP attribute.

Characteristic Implicit Messaging (I/O) Explicit Messaging
Transport UDP TCP
Data Format Only predefined I/O data Commands and structured data
Typical Use Cyclic process control Configuration and diagnostics
Maximum Packet Size Up to 1 472 bytes (Ethernet) / 1 472 bytes Up to 65 535 bytes theoretical, limited by MTU
Connection Unicast or multicast Point-to-point (unicast)

Communication Parameters

Section titled “Communication Parameters”

Connection parameters determine the frequency and transmission trigger of implicit messaging packets. The trigger type can be cyclic, change of state, or on-demand (polled), and the update rate is configured at the connection origin. The infrastructure uses port 44818 for TCP explicit messaging and port 2222 for UDP implicit messaging.

Parameter Typical Value / Range Description
Explicit TCP Port 44818 Used for configuration and programming
Implicit UDP Port 2222 Used for cyclic I/O messaging
Requested Packet Interval (RPI) 0.5 ms to 3 200 ms Implicit data update frequency
Connection Timeout RPI multiplier, typically 4x to 16x Time before declaring connection loss
Standard Ethernet MTU 1 500 bytes Maximum frame size without jumbo frames

Topologies and Physical Media

Section titled “Topologies and Physical Media”

EtherNet/IP operates over standard commercial Ethernet infrastructure, supporting star topology via switches, device-level ring topology (DLR), and linear configuration. The protocol is independent of the physical medium, operating over copper and fiber optic with the same range limitations defined by IEEE 802.3 variants.

Physical Medium Nominal Speed Maximum Distance per Segment
Twisted pair copper Cat5e/6 (100BASE-TX) 100 Mbit/s 100 m / 328 ft
Twisted pair copper Cat6a/7 (1000BASE-T) 1 000 Mbit/s 100 m / 328 ft
Multimode fiber 62.5/125 µm (100BASE-FX) 100 Mbit/s 2 000 m / 6 562 ft
Single-mode fiber 9/125 µm (1000BASE-LX) 1 000 Mbit/s 10 000 m / 32 808 ft

Data Transport Modes

Section titled “Data Transport Modes”

EtherNet/IP adapters can consume and produce data through various transport modes that define how and when packets are sent. The most deterministic mode is cyclic multicast, where the producer emits data to a multicast group at fixed intervals; alternatively, change-of-state (COS) mode transmits only upon a change in I/O data, and polled mode sends data as a response to an explicit request from the scanner.

Transport Mode Transmission Trigger Typical Use
Cyclic Fixed RPI timer High-speed motion control
Change of State (COS) Data change edge Traffic reduction in environments with sporadic changes
Polled (on-demand) Scanner request Diagnostic requests or non-critical events
Multicast Transmission to a group of nodes Distributed I/O control from a single producer

Comparison with Other Industrial Protocols

Section titled “Comparison with Other Industrial Protocols”

EtherNet/IP directly competes with other industrial Ethernet protocols based on IEC 61158 standards. Compared to Modbus TCP, it adds a comprehensive CIP object model and producer/consumer communication; with respect to PROFINET IO, the difference is that EtherNet/IP encapsulates CIP over standard TCP/UDP without requiring a modified media access controller or special hardware for strict real-time.

Protocol Real-time Messaging Transport Ring Topology
EtherNet/IP Implicit UDP + CIP Sync (IEEE 1588) Standard TCP/UDP Native DLR at device level
Modbus TCP Not native (master-slave polling) TCP Dependent on managed switch
PROFINET IO IRT with dedicated hardware or RT with software Proprietary Ethertype MRP (Media Redundancy Protocol)
EtherCAT On-the-fly frame processing Proprietary Ethertype Logical ring with closed port

Frequently Asked Questions (FAQ)

Section titled “Frequently Asked Questions (FAQ)”

What is the difference between implicit and explicit messaging in EtherNet/IP?

Section titled “What is the difference between implicit and explicit messaging in EtherNet/IP?”

Implicit messaging uses UDP to exchange real-time input/output data without embedded commands, while explicit messaging operates over TCP with a request/response structure to read or write parameters, configure devices, and transfer programs.

Which ports should I open in the firewall for an EtherNet/IP network?

Section titled “Which ports should I open in the firewall for an EtherNet/IP network?”

Port 44818 TCP for explicit messaging and port 2222 UDP for implicit messaging must be allowed. Additionally, UDP multicast traffic must be enabled in the IP address range used by the application.

Is it possible to run EtherNet/IP over office Ethernet infrastructure?

Section titled “Is it possible to run EtherNet/IP over office Ethernet infrastructure?”

Yes, EtherNet/IP operates over standard IEEE 802.3 compliant Ethernet hardware, although in office environments it is recommended to segment traffic using VLANs and managed switches that support IGMP snooping to avoid flooding multicast packets across the entire network.

What is CIP and why is it relevant to EtherNet/IP?

Section titled “What is CIP and why is it relevant to EtherNet/IP?”

CIP (Common Industrial Protocol) is an object-oriented application protocol managed by ODVA that provides standardized device profiles and common services across multiple networks, allowing EtherNet/IP to share the same data and configuration model as DeviceNet and ControlNet.

Does EtherNet/IP support ring topology?

Section titled “Does EtherNet/IP support ring topology?”

Yes, the protocol defines DLR (Device Level Ring), a layer 2 redundancy mechanism that allows connecting devices in a ring to maintain communication in the event of a single cable or node failure, with recovery times less than 3 ms for 50 nodes.

Can EtherNet/IP be used with public IP addresses?

Section titled “Can EtherNet/IP be used with public IP addresses?”

Technically it is possible, but it is not recommended for security and determinism reasons. The protocol is designed for isolated or properly segmented control networks, and exposure to the Internet introduces unpredictable latency and vulnerabilities to cyber attacks.

References

Section titled “References”