CIDR Calculator

CIDR (Classless Inter-Domain Routing) replaced classful addressing in 1993, allowing networks to be divided at any bit boundary. The notation IP/prefix (e.g., 10.0.0.0/8) specifies how many leading bits identify the network. A /8 gives 16.7 million addresses, a /24 gives 256, and a /32 gives a single host.

CIDR enables efficient IP allocation. An organization needing 500 addresses can get a /23 (512 addresses) instead of wasting a full Class B. It also supports route aggregation, reducing routing table size across the internet.

The subnet mask defines which bits belong to the network and which to the host. A /24 mask in binary is 11111111.11111111.11111111.00000000 — 24 ones followed by 8 zeros. In dotted decimal that's 255.255.255.0.

Viewing the binary helps understand how addresses are divided. The network address is the IP ANDed with the mask; the broadcast is the network ORed with the wildcard (inverse of the mask). All host addresses fall between these two.

When planning subnets, start with how many hosts each segment needs. A /28 gives 14 usable hosts — suitable for a small office. A /27 gives 30 hosts. A /24 gives 254 — common for department or building networks. Always plan for growth; using a slightly larger prefix than strictly necessary avoids renumbering later.

Private ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) are used for internal networks. CIDR calculators help you carve these into appropriately sized subnets for VLANs, departments, or cloud VPCs.