It allows networks to be subdivided regardless of their traditional class

Class	Network	Subnet Mask	Broadcast
A	17.0.0.0	255.0.0.0	17.255.255.255
B	183.194.0.0	255.255.0.0	183.194.255.255
C	199.83.131.0	255.255.255.0	199.83.131.255

Classless Inter-Domain Routing

CIDR stands for Classless Inter-Domain Routing. It allows networks to be subdivided regardless of their traditional class. These subdivided networks are called subnets. For example, the IP address 121.67.198.94 falls in the Class A network range. By default, the network is 121.0.0.0, the subnet mask is 255.0.0.0, and the broadcast address is 121.255.255.255. However, if you specify a subnet mask, you can alter the portion of the IP address that is used as the network and the portion that is used as the host address. By specifiying a 255.255.255.0 subnet with the 121.67.198.94 address, the network becomes 121.67.198.0 and the broadcast address becomes 121.67.198.255.

Reserved Private Address Space

There are ranges of IP addresses that are dedicated for use in private networks. Youll often see these types of IP addresses being used in your companys internal network and youll most likely being using a range of these IP addresses for your home network as well.

These private addresses are also called non-routable IPs since they are not routed through the public Internet. Youll also hear these IP addresses referred to as RFC1918 address, which refers to the RFC1918 standards document where these private ranges were initially defined.