sibly in several steps) in order to support distribution of the latter. Another approach to vertical partitioning is to sep- arate a given layer according to its internal control structure, provided that resources that exist on a lower level of control must be distributed in a network. We encountered such an example when discussing DNS in Sect. 3.4.5.3. In that analy- sis we saw that DNS is a common distributed resource that offers control points 4 Modeling Vertical and Horizontal Partitions which are used by other layers for address translation (GT-REdef). Thus, layer interfaces between the DNS layer and other layers are interfaces in a control struc- ture, as depicted in Fig. 4.3. Control-structure partitioning (DNS): Resource layer TCP,UDP/IP TCP,UDP/IP Divers connectivity strata n11, n12: Router only n15, n16: DNS server only n13, n18: L2' server with DNS resolver n14, n17: L4' server with DNS resolver TCP,UDP/IP Fig. 4.3 Separating functions of a control structure As long as we do not define separate logical networks for different layers (or group of layers), there is only a single, flat node structure that tends to be more and more complex as we continue to apply VP. This complexity consists not SO much in the connectivity structure of the network, but in an increasing complex structure of allo relations, as Fig. 4.3 indicates. These relations are important since they tell which functional contents a node has or may have. Note, however, that most standards that describe VP structures do not explicitly define allo relations. Instead, complex nodes of a derived network are given spe- cific names that are supposed to associate to their functional contents, e.g., as depicted in Fig. 4.4. The amount of information (indicated by the allo relations of the model in Fig. 4.3) that is implied but not defined when such a simplified model is create is obviously considerable. In general, only the creators of the model in Fig. 4.4 can be expected to understand which assumptions are made by it. Further- more, this approac