Both types exist.
Most USB to RJ45 converters are just that. They don't need any software, as they only convert the stream to run on different media.
Networked USB to ethernet boxes have a processor and need software on the computer to redirect the stream from TCP/IP to serial. They're mostly used for serial stuff, like printers, or a terminal. I don't know of any that work for audio interfaces...
What I use to go long distance with audio, is an RME Digiface. 4 ADAT I/O, headphones out. 32 channels in, 34 out. Optical cable works easily over 100 meter. If that's not enough, go MADI, that goes up to 2 km. Disadvantage: preamp gain is on the ADAT input box, so it's not near the computer.