The present invention discloses methods and systems for implementing inter-network roam, querying and attaching network, wherein the methods and systems are applied in a network where an identity identifier and a location identifier are separate. In the network, an Access Identifier (AID) is used as the user identity identifier of a terminal and a Routing Identifier (RID) is used as the location identifier of the terminal. Said method for implementing inter-network roam includes the following steps: after attaching the Access Service Node (ASN) in a roaming network, a roaming terminal sends a first data packet to a destination device, wherein the destination address of the first data packet is the AID of the destination device; after receiving the first data packet, said ASN in the roaming network queries locally the mapping information of the AID-RID, and requests to query the RID of the destination device to a roaming gateway if the RID of the destination device is not found and the destination device does not belong to the roaming network; the roaming gateway queries the RID of the destination device from the home network of the destination device and returns it to the ASN in the roaming network; according to the RID of the destination device, the ASN in the roaming network sends the following data packets of the roaming terminal to the destination device.