Earlier, it was required to use the same subnet mask across the network. This was called classful networking. With increase in complexity of networks and decrease in available IP addresses it became obvious that classful networking causes waste valuable of IP addresses. To understand how, consider Figure 2-1. The largest subnet requires 30 host addresses. So across the network a mask of /27 is used, which gives 30 hosts per subnet. You will notice that in every subnet except the subnet attached to RouterD, some host addresses will remain unused. In particular, 28 host addresses are wasted for each link between the routers. In total this network wastes 118 addresses and uses 92 addresses.