 this 192.168.1.12/30 192.168.1.16/30 block is 8 subnets of 4 192.168.1.16/30 case, 32 hosts. 192.168.1.20/30 192.168.1.20/30 192.168.1.24/30 hosts each (/30). 192.168.1.24/30 192.168.1.28/30 192.168.1.28/30 192.168.1.32/29 192.168.1.32/29 192.168.1.32/27 192.168.1.40/29 192.168.1.40/29 192.168.1.48/29 192.168.1.48/29 192.168.1.56/29 192.168.1.56/29 192.168.1.64/27 192.168.1.64/27 This single block is 32 192.168.1.64/27 hosts (/27). 192.168.1.96/28 192.168.1.96/28 192.168.1.96/27 192.168.1.112/28 192.168.1.112/28 192.168.1.128/27 192.168.1.128/26 192.168.1.128/26 192.168.1.160/27 This block is invalid. Networks must still 192.168.1.192/29 follow the rules of binary, 192.168.1.192/29 192.168.1.200/29 and as such must begin 192.168.1.192/27 on classful subnet 192.168.1.200/28 192.168.1.208/29 boundaries. Subnets 192.168.1.216/29 may only exist where 192.168.1.216/29 192.168.1.224/29 they would naturally 192.168.1.224/29 192.168.1.224/27 192.168.1.232/29 exist! 192.168.1.232/29 192.168.1.240/29 192.168.1.240/29 192.168.1.248/29 192.168.1.248/29 Figure 34-3. Benefits and limitations of VLSM The benefits of VLSM should be apparent. In the old days, we used to have to put aside entire networks just for /30 subnets (once a /30 subnet was allocated within a classful network, the entire network had to be allocated with the same subnet mask). Often, 80 percent of the network was wasted because it had to be allocated as /30 subnets, when only 10 or SO /30 subnets were needed. With VLSM, you can use 25 percent of a network for /30 subnets, another 25 percent for /27 subnets, and SO on. Needless to say, VLSM has had a serious impact on the allocation of IP space, both on the Internet, and in private networks. If you deal with large numbers of IP networks, like ISPs do, Classless Internet Domain Routing (CIDR) is a most useful tool. While VLSM has had a dramatic impact on IP space allocation within corporate networks, CIDR has had an equally impressive impact on public Internet networks allocated to ISPs. CIDR is sort of the i