nw1 from using an unreliable hosting-network service. In such cases the common layer will handle a service-enhancing proto- nw01 or nw02 may be synchronous, and the common layer may create an asyn- chronous service for its hosted networks. This requires the common layer to handle a framing protocol (such as in MTP L2). It may also be the other way around (as in ATM/AAL1 circuit-emulation). It may protect hosted networks in nw1 from changes of connectivity type. For example, in the case where nw01 or nw02 is a connection-oriented network, the common layer in nw1 may create a connection-less service for its hosted net- works. This also requires the common layer to handle a protocol. If we transform nw1 to a hosting network, we have to define such a common layer, as Fig. 3.12 shows. The common layer element in node n11 is part of the common layer in nw1. Note that nodes of hosted networks are volatile entities in the model in relation to the nw1 itself. nw2 and nw3 nodes may appear, disappear, or be reconfigurated in the nw1 network at any time. The way configuration tables in nw1 look depend on how nw1 supports discrim- ination and address translation. Figure 3.13 gives an example. This set of tables is based on the following requirements: 1. We assume that nw1 uses discrimination by the layer APi method, according to Sect. 2.2.2 (which implies that layer elements of the same layer all are associ- ated to the same APi value). This problem is solved by realizing a table called T-AP in the common element that relates an APi value to a hosted logical-network identifier (NWi). 2. We assume that the common element allows hosted nodes to use their own num- bering system for addressing, i.e., node names of networks nw2 and nw3, respectively. When these names are used in the layer interfaces, the common element regards them a global titles (GT). This problem is solved by adding another table, the GT translating table, T-GT. This table defines the translation of a global title to a route-endpoint identifier (REi). The REi i