Understanding the CIDR notation

As I mentioned earlier, the concept of classful subnetting isn't used that often anymore. The main use of classful subnetting is in the default configuration of network appliances (such as routers) and also the default settings of most DHCP servers. In the case of home routers, the DHCP server is typically built in, and the default scheme is most often a Class C network (typically 192.168.1.0, with a couple of variations in between). But with most devices, home or enterprise, you'll probably get a Class C IP scheme if you don't change it to something else. There's nothing necessarily wrong with these default settings in a small network, but almost no one configuring a network nowadays uses the classful style. The reason for this is that classful networks are too limiting; in complex network roll-outs, it can be a pain to try to force your network plan to fit within one of these predetermined schemes.

The answer to the lack of flexibility in classful schemes...