A method (200) for selecting a core network for a communication device includes the steps of: receiving (220) at least one core network identifier; selecting (230) a core network identifier; setting (215) an indicator to indicate whether a substitute core network is allowed; determining (240) whether the selected core network identifier corresponds to a shared network; forming a registration request message containing the selected core network identifier; including (243) the indicator in the registration request message; and transmitting the registration request message.