Abstract: A method for controlling number of addresses in an address table of a switch in a network system, the method comprising the steps of adding a control mechanism in a learning mode of the switch; enabling one of a plurality of ports of the switch to detect a number of learned addresses in the address table; and locking the address table to stop learning and discard subsequently received packets having unmatched addresses if the number of learned addresses being detected has exceeded a predetermined maximum number of learnable addresses of the address table, thereby preventing the address table being completely occupied by a number of abnormal packets.