Hi Dragon,
After a google search, the cable seems to be for enabling a PC without an ethernet port to have one, using the serial port. (maybe like an external LAN card?)
The thing that puzzles me though is the male RJ-45 connector, surely it would be a female one, to accept an RJ-45 cable, or maybe the device plugs directly into the socket on the router/modem?
One of the devices I found could be used this way, as it was for cash registers. The ethernet end plugs into a computer and becomes a virtual COM port. The other end of the cable becomes a standard serial port. Is this what you meant?
Maybe I'm getting confused
Hope I was of some assistance.
-HyperNerd