What Is CIDR Notation?
CIDR (Classless Inter-Domain Routing) notation is a compact way to represent an IP address and its associated network mask. It is written as an IP address followed by a slash and a number (the prefix length) - For example, 192.168.1.0/24. CIDR replaced the older class-based system (Class A, B, C) in 1993 and is the standard for all modern IP address allocation and routing. Use our CIDR calculator to expand any notation into its full host range.
Reading CIDR Notation
| CIDR | Network Address | Subnet Mask | Prefix Bits | Usable Hosts |
|---|---|---|---|---|
| 10.0.0.0/8 | 10.0.0.0 | 255.0.0.0 | 8 bits fixed | 16,777,214 |
| 172.16.0.0/12 | 172.16.0.0 | 255.240.0.0 | 12 bits fixed | 1,048,574 |
| 192.168.1.0/24 | 192.168.1.0 | 255.255.255.0 | 24 bits fixed | 254 |
| 192.168.1.64/26 | 192.168.1.64 | 255.255.255.192 | 26 bits fixed | 62 |
| 203.0.113.5/32 | 203.0.113.5 | 255.255.255.255 | 32 bits fixed | 1 (host route) |
| 0.0.0.0/0 | 0.0.0.0 | 0.0.0.0 | 0 bits fixed | All IPs (default route) |
How the Prefix Length Works
The number after the slash tells you how many bits of the 32-bit IPv4 address are fixed (the network portion). The remaining bits are free to vary (the host portion). A /24 fixes the first 24 bits, leaving 8 bits for hosts - That's 2⁸ = 256 total addresses, minus 2 reserved = 254 usable hosts. A /25 fixes 25 bits, leaving 7 bits = 128 total, 126 usable.
Where You'll Encounter CIDR Notation
- Firewall rules and access control lists - E.g. "allow 10.0.0.0/8" means allow any IP in the entire 10.x.x.x range.
- Cloud provider security groups (AWS, GCP, Azure) use CIDR to specify which IPs can connect to resources.
- VPN split-tunneling configuration - Specify which CIDRs to route through the tunnel vs. send directly.
- BGP routing tables - Internet routers exchange CIDR prefixes to describe which networks they can reach. See a live example via our ASN lookup.
- WHOIS and RIR records - IP address allocations are listed in CIDR format.