Classless Inter-Domain Routing (CIDR) is a method used to allocate and manage IP addresses more efficiently than the traditional class-based system. It allows for variable-length subnet masking, which means that networks can be divided into smaller or larger segments based on actual needs rather than fixed sizes. This flexibility helps reduce the wastage of IP addresses. Introduced in the 1990s, CIDR replaced the older classful addressing system, which divided IP addresses into classes A, B, and C. By using a notation called "slash notation," such as /24, CIDR simplifies routing and improves the scalability of the Internet, allowing for better management of the growing number of devices connected to it.