They are two similar modules using the same Modbus TCP/IP protocol; but are intended for slightly different applications.
The MNET has both server and client drivers and functionality. The MNETC has only client functionality. The distinction lies in who creates messages on a Modbus TCP/IP network and who responds to those messages. Clients create messages and expect responses. Servers listen for messages addressed to them and respond to whatever client sent the message.
The MNET is a more general-purpose module, in that it can act like a server but also be a client. The MNET server will simultaneously support connections with up to 5 mbap Clients on service port 502 and 5 MNET clients on service port 2000. The MNET also includes one client that can send messages to one or more servers.
The MNETC is more of a special-purpose module, in that it can only act as a client. The MNETC has no server built-in. it cannot be configured to wait for and respond to messages on the network. It can only act as a client and create messages on the network. The advantage of the MNETC over the MNET for client applications is that, while the MNET contains one client, the MNETC contains 30 clients. This means that, while the MNET can connect to multiple servers only one at a time, the MNETC can connect to up to 30 servers simultaneously.
in certain applications, some server devices do not respond well to clients that constantly open and close connections. These devices expect a client to open a connection to their server and keep that connection open. The MNET can do this if it is only configured to create messages to one server; but, if it must message more than one server, it must constantly open and close connections to each, in turn. in these situations, the MNETC is a better choice because a user may configure individual client.