There are provided a method, a network system, and a router capable of decreasing the chance of VRM or VLM changes so as to reduce signaling, and consequently contributing to better scalability. The method is applied to mobility management used in an IP-based network system, which contains a plurality of access routers (AR), location managers (LM) and routing managers (RM), through which a mobile node (MN) transmits and receives information. According to the method, a visited routing manager (VRM) where the MN is currently located monitors the MN, an AR currently communicating with the MN discriminates whether or not the MN is moving out of its connectivity range, and the VRM discriminates whether or not the MN is moving out of its control range. Then, the method switches mobility management over from the current AR to a new AR in accordance with the discrimination results, and switches mobility management over from the current VRM to a new VRM in accordance with the discrimination result.