A method is provided for use in a Mobile IP network in which it is determined whether a Mobile Node (10) in a visited network is reachable on a new claimed Care-of Address for the Mobile Node (10) using information relating to a pre-established cryptographic relationship between the Mobile Node (10) and an Access Router (20) of the visited network. It may be determined, through communication between a Home Agent (30) for the Mobile Node (10) in the Mobile Node 10's home network and the Access Router (20), whether such a pre-established cryptographic relationship exists. The existence of such a pre-established relationship would indicate that the Mobile Node (10) is reachable on the claimed Care-of Address.