 we will discuss in this section is the subnet concept. This con- cept is not defined by IP, but is a method (described in IP literature) to generate host networks that are subsets of class-A and class-B host networks. We will base this discussion on the model in Fig. 3.81, which shows the network structure of an IP network that comprises three class-C host networks, identified as nn1 through nn3. The IP network operator has installed a new set of hosts, nn3. nn2/hni nn2/hn2nn2/hn3 nn1/hn? nn3/hn1 nn1/hn2 nn3/hn2 nn1/hn nn3/hn3 Fig. 3.81 A part of an IP network with three class-C host network As was said before, network numbers must be registered centrally for a host net- work (there is an administrative function within ISOC for that). Once a new net- work number has been assigned, routers worldwide must be informed about the new host network. There are a number of problems associated with this procedure. In many cases, therefore, a host-network operator would like to assign network 3 Logical Network Modelling numbers himself when he installs a new set of hosts. This must, however, be done in a way that allow the rest of the world still to route packets to all hosts. The method devised creates subnets within a host network by using a part of the host number field in IP addresses as a subnet number (snn). This technique is only relevant for use on class-A and class-B host networks. If the operator for the net- work in Fig. 3.81 had instead used a class-B network number for all his hosts from the beginning, he could have extended his network at will by installing new sub- nets, without having to announce these subnets to the rest of the world. Figure 3.82 demonstrates this alternative solution. IP-address format for class B host-networks: Format identifier' Network number Host number nn1 1 hn0-hn65 535 Subnet Subnet host number number snn0- hn0-hn4095 snn15 nn1/hn4 nn1/hn5 nn1/hn6 sn2/hn1 snn2/hn2 sn2/hn3 nn1/hn3 nn1/hn7 sn1/hn3 sn3/hn1 nn1/hn2 nn1/hn8 sn1/hn2 sn3/hn2 nn1/hn1 nn1/hn9 sn1/hn1 sn3/hn3 nn1/hn6 Network