A network system for communicating with mobile nodes has multiple gateways that can operate with each using a different access technology. In an IPv6 system, the access gateways each provide an interface identifier to the mobile nodes using the same one or more keys and hash so that different the same interface identifier regardless of the access technology used. The access gateways can have multiple options for using keys and/or hashes to provide an interface identifier such that the method for determining the interface identifier is configurable.