You can use the Siemens NetPro configuration program to define a so called S7-LINK between a Citect station and a CP 433-1 module.  There you define the details for the Ethernet connection, like IP address etc..

For the S7-300 PLC you can use a CP 343-1 module or a CP343-1 TCP module.

If you try to do the same thing as for the S7-400 you will get an error message, saying: 

"the CPU S7-300 does not support S7 LINK type" 

or

"the remote partner does support the S7-functions or does not support bi-directional S7 LINK type".

The solution is to not define the Citect station (PC) in the NetPro utility and simply edit the CP 343-1 module properties and define the IP address.

In Citect the S7NT driver is used to communicate over Ethernet with either the S7-300 (via CP 343-1 module) or the S7-400 (via CP 443-1 module).  In the PC you can use a standard Ethernet card and the Siemens Softnet S7 software.  You configure the communication settings for the S7 communication in the Siemens COML S7 and the Siemens "Setting the PG/PC interface" utilities.