An Ethernet switch having a function of expanding a port, apparatus and method for expanding a port are provided. A receiver receives a frame including predetermined port identification information. An information detector detects the predetermined port identification information and a terminal address of a terminal connected to a sub port corresponding to the predetermined port identification information. A storage unit stores an address table including the terminal address and the predetermined port identification information. A reader reads port identification information corresponding to a destination address included in the frame from the address table. A frame transformer adds the read port identification information to the frame. An output unit outputs the frame transformed by the frame transformer to a main port connected to a sub port corresponding to the read port identification information among main ports that can be connected to at least one sub port.