When going down into Ethernet or any other signal, you find that there is nothing more than ones and zeros. So practically any programmable device theoretically can send any information in any format. The only limit is RAM space, speed and signal conditioning. Cnlohr was able to squeeze Ethernet functionality in to small Attiny85 microcontroller. He was able to do so without external circuits – twisted pair was directly attached to microcontroller pins. This is really dangerous way to do, because Ethernet signal this way is forced to be tied to mcu ground. Ifthere is a DC offset in the line, it can cause damage. Normally there is a magnetics involved in signal line.
Anyway this is great hack showing that anything is possible with minimal component count and small processing power. Attiny is clocked at 20Mhz and uses one twisted pair for network connection. IT is recognized as 10MBit Ethernet switch that can send hard-coded packets once one of couple buttons is pressed. To get 10MBit from 20MHz micro thee is an assembly language involved. As it is said, this project is more for playing but strictly not for production as it is dangerous and not reliable.