Method and nodes for selecting a network prefix to be advertised in an IP network comprising an access router and a mobile node (MN). From the access router, a first validity period is used to advertise at least a first and a second network prefixes towards the MN. The access router receives a message from the MN indicating an intended address thereof that it intends to use. In the access router, based on the message, only one of the first or second network prefix that the MN needs to use is then determined. The access router afterwards advertises towards the MN the only one network prefix with a second validity period longer than the first validity period. The access router may further acquire a current address and a current network prefix used by the MN and it may use these in the determination of the only one network prefix.