What is CIDR?
CIDR (Classless Inter-Domain Routing) replaced the old Class A/B/C addressing system in 1993. It uses a suffix notation like /24 to specify how many bits of the 32-bit IPv4 address define the network portion — giving flexible network sizes instead of fixed class blocks.
Reading CIDR Notation
192.168.1.0/24 means: The first 24 bits (192.168.1) are the network address. The remaining 8 bits (0–255) are the host portion. Total addresses: 2^8 = 256. Usable hosts: 254 (minus network address .0 and broadcast .255). The subnet mask is 255.255.255.0.
Common CIDR Blocks Reference
/8: 16,777,216 addresses (Class A replacement). /16: 65,536 addresses. /24: 256 addresses — the most common for small networks. /27: 32 addresses (30 usable) — common for small VLAN segments. /30: 4 addresses (2 usable) — point-to-point links. /32: 1 address — single host route.
Subnetting a Network
Example: You have 192.168.1.0/24 and need 4 subnets with ~60 hosts each. Borrow 2 bits from the host portion → /26 (64 addresses each, 62 usable). Result: 192.168.1.0/26, 192.168.1.64/26, 192.168.1.128/26, 192.168.1.192/26.