A system for providing power over data lines, particularly Ethernet data lines, comprises a network switch including a multiplicity of ports each of which is capable of supplying power in addition to data packets, a controllable power source which is coupled to said ports for supplying power thereto and a processor which is coupled to control the supply of power by the power source. The processor is programmed to guarantee the supply of power to selected ports and to allow or inhibit the supply of power to ports other than the selected ports, having regard to a specified limit on the supply of power by the controllable power source and the total guaranteed power to the selected ports.