RS-232 is a means of transmitting serial data through a cable. In order to go from RS-232 to TCP/IP, you would need a microprocessor. It would input the RS-232 data, and send it as data packets according to the TCP/IP protocol. Here is a link to the Wikipedia article on TCP/IP - http://en.wikipedia.org/wiki/Internet_protocol_suite.
No such thing exists as a purely hardware implementation.
It might be possible to conceive of a processor with a UART connected to an RS232 transceiver, and a 10/100 Ethernet MAC connected to some magnetics. The software stack would include TCP/IP on the Ethernet side and maybe SLIP (Serial Link Internet Protocol - RFC 1055, Romkey, J. 1988) on the RS232 side. Is that what we are talking about?