68.1.208/29 boundaries. Subnets 192.168.1.216/29 may only exist where 192.168.1.216/29 192.168.1.224/29 they would naturally 192.168.1.224/29 192.168.1.232/29 exist! 192.168.1.224/27 192.168.1.232/29 192.168.1.240/29 192.168.1.240/29 192.168.1.248/29 192.168.1.248/29 Figure 36-3. Correct and incorrect subnets using VLSM 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 im- pact on public Internet networks allocated to ISPs. CIDR is sort of the inverse of VLSM: whereas VLSM prescribes rules for subdividing networks, CIDR prescribes rules for referencing groups of networks with a single route statement. Aggregating routes may seem like a solution looking for a problem if you've only ever dealt with small or medium-size corporate networks, but, rest assured, it provides a real benefit. Usually, small or medium-size companies use one of the private IP net- works described by RFC 1918. If a company uses the entire 10.0.0.0/8 network and subdivides it to maximize efficiency, each of these subdivisions is technically a subnet. While VLSM deals with subnets, CIDR deals with groups of major or classful networks. Chapter 36: IP Design Figure 36-4 shows how a single route statement can reference 16 Class C networks. The route is called an aggregate route or a summary route. 192.168.0.0/24 192.168.1.0/24 192.168.2.0/24 192.168.3.0/24 192.168.4.0/24 192.168.5.0/24 Even though these are natively Class C networks, 192.168.6.0/24 by using CIDR, we can reference them all with a single route entry. 192.168.7.0/24 192.168.0.0/20 192.168.8.0/24 192.168.9.0/24 192.168.10.0/24 192.168.11.0/24 192.168.12.0/24 192.168.13.0/24 192.168.14.0/24 192.168.15.0/24 Figure 36-4. CIDR route aggregation On Internet-attached routers with full tables, the routing tables may contain hundreds of thousands of routes. Anyone can see them at any time by connect