You need to clearly define what you mean by TCP/IP since it is only a transport protocol. You should specify the application protocol. Are you talking about Ethernet/IP or Modbus/TCP or something else?
I think that the cheapest way to solve your problem would be to get a gateway. I also believe that the gateway solution will require a minimum amount of work. There are several suppliers out there. I have only experience with the Anybus X-gateway.
In my opinion ControlNet is a great peer to peer and IO network for the Logix platform, but for HMI's and Scada I would go Ethernet. With workstations the choice with IO servers that support ControlNet is limited whereas nearly every Scada package or IO server provider will have a ControlLogix packages.
> Which converter you suggest for controlnet to Modbus/TCP <
Just installed a module to send words from a AB compact Logix to a SCADAPAK which uses Modbus/TCP and it worked prefect. Made by RTA,Inc. (Real Time Automation).