What is Classless Inter-Domain Routing (CIDR)?
**CIDR** is an IP addressing method introduced in 1993 that replaced the old class-based system (Class A, B, C subnets). Under CIDR, an IP address is appended with a slash followed by a prefix length (e.g. `/24`), indicating the number of bits allocated for the netmask.
For example, in a `/24` prefix:
- The first 24 bits are dedicated to network routing
- The remaining 8 bits are left for local device assignment
- This supports exactly 256 unique addresses
- Subtracting 2 (one for the Network address and one for the Broadcast address) leaves 254 Usable IP hosts
Network engineers use subnet calculations to logically isolate network clusters, enforce security access policies, and avoid conflicts across routed nodes.