ed with the subnet concept. I therefore refer to host network, when I mean a set of hosts that have the same network number. 3.4 Modelling Telecom Resource Systems network of Fig. 3.75, the routing tables become modified accordingly, see Fig. 3.78. To structure host identifications into a network number and a host number sim- plifies routing, since routers that do not connect hosts can base their routing entirely on network numbers. Number administration is also made much easier. Instead of managing billions of host numbers centrally, only network numbers must be managed that way. An administration that has a network number can assign host numbers at will. As a consequence, two routing tables are managed by RAs: a table based on network numbers, and a table based on host numbers. Only routers that access hosts need the latter. Note that the concepts host and host network do not identify physical nodes or physical networks. Neither do they identify external transport networks. A host network defines a subset of all hosts that are served by an IP network, and a host is any physical node that has an IP address (although its main tasks might be things other than IP communication). An IP network is often characterized as a "network of networks". I must admit that it took me some time to discover that the referred networks in this expression were not a set of hosts that had the same network number, but the external transport networks that were used for access between hosts and routers. Although there may be a one-to-one correspondence between a set of hosts and some external transport network, IP does not require that. hosts with the same network number may be interconnected to the rest of the IP network over different external transport networks. See e.g. the hosts in nn1 in Fig. 3.78: hn1 and hn2 are accessed over an Ethernet, while hn3 and hn4 are accessed over the ISDN packet access service, but they all have the same network number. In order to economize with IP addresses, and keep costs for central number adminis