CIDR Range Tool
Expand any CIDR block to start/end IPs, total addresses, and usable host count.
What CIDR notation tells you
CIDR (Classless Inter-Domain Routing) replaced the old class A/B/C system. Instead of being limited to /8, /16, or /24 boundaries, CIDR lets you size subnets to fit. Each "prefix length" specifies how many bits identify the network — and the rest are available for hosts.
Block sizes by prefix
| Prefix | Subnet mask | Total addresses | Common use |
|---|---|---|---|
| /8 | 255.0.0.0 | 16,777,216 | 10.0.0.0/8 (largest RFC 1918 block) |
| /12 | 255.240.0.0 | 1,048,576 | 172.16.0.0/12 (mid RFC 1918 block) |
| /16 | 255.255.0.0 | 65,536 | 192.168.0.0/16, large enterprise |
| /24 | 255.255.255.0 | 256 | Standard home/office subnet |
| /29 | 255.255.255.248 | 8 | Small ISP customer block |
| /30 | 255.255.255.252 | 4 | Point-to-point links |