CIDR

Classless Inter-Domain Routing replaced the old Class A/B/C system by allowing any prefix length from /0 to /32, giving engineers precise control over how address space is divided. A block like 192.168.10.0/26 allocates exactly 64 addresses (62 usable) instead of forcing a full Class C. This efficiency helped grow the internet by enabling route summarization, which shrinks routing tables.

The key exam point is that CIDR notation and subnet masks express the same information differently, so /26 equals 255.255.255.192. Variable-length subnet masking (VLSM) relies on CIDR, letting differently-sized subnets be carved from one address block.