A port number extension method and a switch are provided. A first switch determines bits for identifying the first switch in a Media Access Control (MAC) address in an identifier (ID) of a specified bridge; calculates a bit for port number extension according to the bits for identifying the first switch; and combines the bit for port number extension and original bits for specifying a port, to obtain new bits for specifying a port. In this way, a problem that Spanning Tree Protocol (STP) port numbers are insufficient in a super virtual fabric with a massive quantity of ports is resolved.