The physical principle of broadcast and multicast is given. When Alice needs to send out an information, she sends out a conjunction claim signal to the switch that connects with hers. After receiving the conjunction claim the signal, the switch pass to examines the purpose address to judge whether it is a point-to-point communication or broadcast and multicast. If the purpose address is A or B or C address, then the switch carries on the correspondence operation of point-to-point; if the purpose address is a D address or an address (local or appointed network webcasting) with special meaning, then the switch carries on the correspondence operation of broadcast and multicast. With broadcast and multicast, the fidelity decreases with the increase of receiving terminal number, and its limit is 2/3.