CJ1W-ETN21 is an Ethernet/IP communication module manufactured by Omron for its CJ-series PLCs. It provides Ethernet connectivity and allows the PLC to communicate with other devices and systems that support the Ethernet/IP protocol. The module supports various communication modes, such as TCP/IP socket services and UDP/IP services, and can be used for both program control and data exchange between the PLC and external devices. It supports up to 16 socket connections and has built-in diagnostic and error detection functions to ensure reliable communication.