The Ethernet extension system contains at least a signal and power conversion device, a conversion device, and a coaxial cable connected in between. The signal and power conversion device receives baseband network signal and electricity in accordance with PoE, and delivers the baseband network signal and electricity to the conversion device through the coaxial cable. The conversion device then provides the baseband network signal to an ordinary Ethernet network device. Baseband network signal and electricity as such could be delivered through a coaxial cable beyond the conventional Ethernet without additional configuration of power supplies or power sources, and without repeatedly modulation and demodulation, coding and decoding, or conversion of communication protocols.