CIDR stands for Classless Inter-Domain Routing. It is a method for dividing IP address space into subnets to prevent exhausting IPs. Instead of using the traditional classful system with A, B, and C classes, CIDR allows adding subnet masks in IP addresses.
One CIDR notation example is 10.0.100.25/19, where subnet mask 19 is the number of bits used for the network address. The rest of the 32 bits are used for the host address. This classless system is efficient when managing large networks since IP addresses are not wasted.