At the lower left we used the method to draw a hosted node inside its hosting node to show the same thing. This method may seem attractive in this case. However, in more realistic cases, it can create models that are SO cluttered with route symbols and annotations that the model is not intelligible. The lower right corner of Fig. 3.11 shows a table representation. Translating a logical network (here nw1) to a common hosting network requires us to extract such elements out of the original layers of nw1 that can be useful for many different functions, and put them in a common layer that interfaces all host- ing networks and has a standardized and manageable layer interface to other func- tions/layers in nw1, as shown in Fig. 3.12. The model shows one of the nw1 nodes, n11, in the L-N plane. The internal structure can be directly derived from the logi- cal-network structure of Fig. 3.11. nw2 route Layer nw2 routes nw3 route Layer element element nw3 route Standardized and manageable layer interfaces n11 common nw1 routes nw1 route layer element nw01 layer nw02 layer element element NOnw1 Fig. 3.12 A hosting node of nw1 The actual properties and design of such a common layer varies a lot in existing network systems. For nw1 one can imagine any or all of these functions of a com- mon layer: A common layer may allow nw1 to act as a hosting network for a number of hosted networks that operate concurrently. In that case the common layer must perform some form of discrimination, as was discussed in Sect. 2.2. 3 Node Structures It may allow networks that are hosted in nw1 to use their own numbering system for addressing, i.e., making them independent of whatever addressing parame- ters the hosting network uses. It may protect hosted networks in nw1 from any changes of layer-interface prim- itives in case any of the hosting networks nw01 or nw02 are replaced or modi- fied. This requires a modification of the common layer, but not of layer interfaces that are used by hosted networks in nw1. It may protect hosted networks in 