Suppose that network addresses are scarce and are assigned so that they are not globally unique: in particular, suppose that the some block of addresses may be assigned to multiple organizations. How can the organization use these addresses? Can users from two such organizations communicate with each other?

To make the example concrete suppose that two organizations are assigned the same set of telephone numbers. Clearly, users within each organization can communicate with each other as long as they have a unique address within the organization. However, communications outside an organization poses a problem since any given address is no longer unique across multiple organizations.

A possible approach to enabling communications between users in different organizations is to use a two-step procedure as follows. Each organization has a special gateway to communicate outside the organization. Internal users contact the gateway to establish calls to other organizations. Gateways have procedures to establish connections with each other. This enables gateways to establish connections between their internal users and users in other networks