Abstract: VLAN is a new research method on computer networks, which can increase flexibility of configuration and management for networks. The paper here gives something about VLAN topology discovery. It first analysises a case, and then proposes an algorithm and data structure about topology. The algorithm is based on the platform of Windows/NT , VC++5 0, WinSock2 0, and WinSNMP2 0. It would be beneficial and advisable for developing a network management system software.