 table Subnet table Route Transport Route Transport snn table Fig. 3.82 A logical network model of subnets within a class-B host network The figure shows at the top the class-B format. The network number comprises 14 bits, and the host number 16 bits. The wise operator has split the host number into a subnet field of 4 bits (16 subnets), and a subnet host number of 12 bits (4096 hosts per subnet). Presently, his logical network looks as in the lower part of Fig. 3.82. From the outside it is a class-B network, with network number nn1, contain- ing up to 65536 hosts. The network consists presently of three subnets (sn1 through sn3; sn3 is the last subnet the operator installed). Each of the subnets can hold up to 4096 hosts. 3.5 Logical Network Structures The effect of using subnet numbers is that operators not need announce new subnets to the rest of the world. Instead, each host within the operator network gets two IP addresses: one that is used in communication with the outside world (as nn1/hn6), one that is used for routing inside the operators network (sn2/hn3). The necessary translation between these addresses requires that routers include a subnet table as well, see Fig. 3.82. Adding a new subnet (sn3 in the model) only requires the subnet tables updating. Logical Network Structures 3.5.1 Introduction Chapter 3 has SO far demonstrated that any kind of communication service provider (CSP) can be modelled as a logical structure, using only a few types of generic modelling units. We have also seen that all CSPs can be modelled according to the processing system model we introduced in Chaps. 1 and 2. This model says that there are three major logical parts in CSP-models: a resource system, a process control system and CSP users. The modelling units we have identified can be clas- sified accordingly. Note, however, that we are not specifically interested in CSP users, since they exist outside models. We can divide the units into primitive units, composed units and abstracted units. Section 3.5.2 provides a syste