A method of operating a bridge node (B.sub.0) in a network system. The bridge node comprises a plurality of ports (BP.sub.0.x). The method comprises a step of receiving a frame (240), from a device in the network system and other than the bridge node, at a port in the plurality of ports. The frame comprises a source network address. The method is also responsive to at least one condition (250, 260) associated with the port in that the method stores the source address in a forwarding table associated with the bridge if the at least one condition is satisfied. The at least one condition comprises whether the frame was received within a time window T.sub.w of when a threshold number of previous frames were received at the port and their respective source network addresses were stored in the table.