Definition
CIDR, or Classless Inter-Domain Routing, is a method for allocating IP addresses and routing IP packets more efficiently than older classful systems. It allows for the creation of IP address blocks of arbitrary size, unlike the fixed sizes of Class A, B, and C networks. CIDR notation uses a slash followed by a number to indicate the number of bits in the network portion of an IP address, enabling flexible subnetting. This flexibility is crucial for network administrators to manage IP address space effectively and reduce the size of routing tables. For instance, a network administrator might use 192.168.1.0/24 to represent a local network of 256 IP addresses. It is often confused with subnet masks, which are a component of CIDR but not the entire concept. Understanding CIDR is vital for network security and architecture roles and appears on exams like Security+, Network+, CySA+, CISSP, and CCSP.