 ATM, ITU-T X.121 for X.25. IP- networks and LANs have their own addressing systems. 3 Logical Network Modelling The structure of addresses is an important issue for all classes of WANs, since it determines how routing analysers inside the network will work, the size of routing tables a switch needs to work with and the costs for keeping routing tables updated. For example, if we consider the IP network in Fig. 3.75, it is obvious that having an unstructured IP address would be devastating to the concept of Internet as a net- work that grows incrementally, that has a minimum of number administration and that can be implemented with rather cheap routers. Unless the IP address is struc- tured, many routers must have an entry for every host in the IP network (billions), and every new installed host must be registered by a central authority, which also has to inform thousands of routers about the new host. Most numbering plans in public networks are quite sophisticated, defining address structures on several levels (international, national, regional, local). IP has selected a rather flat address structure, where an IP address is a concatenation of a network number (nn) and a host number (hn). nn1/hn4 nn1/hn3 nn2/hn1 nn1/hn2 nn2/hn2 nn1/hn1 Network table Route Transport Route Transport hn-table hn-table AAL3/4U AAL3/4U others AAL3/4U others AAL3/4U Host table Route Transport Route Transport Fig. 3.78 The IP address structure implies two types of routing tables, a network table and a host table This means that the set of all hosts in an IP network is partitioned into subsets that are identified with network numbers. Within each subset, the hosts are num- bered independently from other subsets of hosts17. If we apply this principle to the I prefer to call the set of all host in a network for the IP network, and therefore call the subset of hosts that have the same network number an IP-subnetwork with the corresponding subnet- work number. This is, however, not in alignment with current literature and may become con- fus