The network to which the mobile node is attached when it is not attached to its home network, and on which the care-of-address is reachable from the rest of the Internet.

In Diameter Mobile IP applications, service for different geographical areas may be provided by different organizations. To provide uninterrupted service to a Mobile IP user, service delivery may be provided by networks belonging to several organizations as the user transfer travels through different areas. When a mobile node is in an area that is not controlled by the user's organization, the user is in a foreign network (out-of-network).

Any network other than the home network of the mobile node.