What is Classless Inter-Domain Routing (CIDR) used for?

Classless Inter-Domain Routing (CIDR) is primarily used for replacing the fixed IP class system that was previously in use. The conventional class-based system (Classes A, B, and C) divided the IP address space into predefined blocks, which often resulted in inefficient use of IP addresses. CIDR introduced a more flexible method of IP address allocation by allowing for variable-length subnet masking (VLSM). This means that networks can be divided into smaller or larger subnets as needed, leading to more efficient use of address space and improved routing.

CIDR notation expresses IP addresses in a more usable form, combining the IP address with a prefix length that indicates the number of bits used for the network portion. For example, an address might be represented as 192.168.0.0/24, where "/24" signifies that the first 24 bits are used for the network address, allowing for a more precise allocation of addresses based on the actual needs of a network rather than adhering to the rigid classes.

Understanding CIDR is crucial for network design and management, as it enhances scalability and supports the growth of the internet by allowing for more efficient routing and address utilization.