CIDR Calculator
Classless Inter-Domain Routing (CIDR) notation expresses an IP address and its routing prefix together. Enter a CIDR block (e.g. 10.0.0.0/8) to compute all related network parameters.
CIDR Notation Explained
CIDR (Classless Inter-Domain Routing) replaced the old class-based IP addressing system in 1993, enabling more efficient allocation of IPv4 address space. The prefix length - The number after the slash - Indicates how many leading bits identify the network portion. The remaining bits identify individual hosts within that network.
Subnetting vs Supernetting
Subnetting splits a larger network into smaller subnetworks by extending the prefix length (e.g. dividing a /24 into two /25s). Supernetting (route aggregation) does the opposite - It combines multiple smaller networks into a larger summary route with a shorter prefix, reducing the number of entries in routing tables. Both use the same CIDR math.
Complete CIDR Reference Table
| Prefix | Subnet Mask | Total Addresses | Usable Hosts | Subnets of /24 |
|---|---|---|---|---|
| /8 | 255.0.0.0 | 16,777,216 | 16,777,214 | 65,536 |
| /9 | 255.128.0.0 | 8,388,608 | 8,388,606 | 32,768 |
| /10 | 255.192.0.0 | 4,194,304 | 4,194,302 | 16,384 |
| /11 | 255.224.0.0 | 2,097,152 | 2,097,150 | 8,192 |
| /12 | 255.240.0.0 | 1,048,576 | 1,048,574 | 4,096 |
| /13 | 255.248.0.0 | 524,288 | 524,286 | 2,048 |
| /14 | 255.252.0.0 | 262,144 | 262,142 | 1,024 |
| /15 | 255.254.0.0 | 131,072 | 131,070 | 512 |
| /16 | 255.255.0.0 | 65,536 | 65,534 | 256 |
| /17 | 255.255.128.0 | 32,768 | 32,766 | 128 |
| /18 | 255.255.192.0 | 16,384 | 16,382 | 64 |
| /19 | 255.255.224.0 | 8,192 | 8,190 | 32 |
| /20 | 255.255.240.0 | 4,096 | 4,094 | 16 |
| /21 | 255.255.248.0 | 2,048 | 2,046 | 8 |
| /22 | 255.255.252.0 | 1,024 | 1,022 | 4 |
| /23 | 255.255.254.0 | 512 | 510 | 2 |
| /24 | 255.255.255.0 | 256 | 254 | 1 |
| /25 | 255.255.255.128 | 128 | 126 | - |
| /26 | 255.255.255.192 | 64 | 62 | - |
| /27 | 255.255.255.224 | 32 | 30 | - |
| /28 | 255.255.255.240 | 16 | 14 | - |
| /29 | 255.255.255.248 | 8 | 6 | - |
| /30 | 255.255.255.252 | 4 | 2 | - |
| /31 | 255.255.255.254 | 2 | 2 | - |
| /32 | 255.255.255.255 | 1 | 1 | - |
Address Space by Prefix Length
Relative address space size (logarithmic scale)
Frequently Asked Questions
The formula is: hosts = 2^(32 - prefix_length) - 2. For a /24: 2^(32-24) - 2 = 256 - 2 = 254. The -2 accounts for the network address and broadcast address. For /31 and /32 the standard rules do not apply - /31 is used for point-to-point links (RFC 3021) and /32 represents a single host.
Classful addressing divided IPv4 space into fixed classes: Class A (/8), Class B (/16), Class C (/24). This was wasteful - A company needing 300 IPs had to receive a full /16 (65K addresses). CIDR allows any prefix length, so that company can receive a /23 (512 addresses) instead, preserving scarce IPv4 space.
Route aggregation (supernetting) combines multiple consecutive CIDR blocks into a single shorter-prefix route. For example, four /24 networks can be summarized as one /22. This reduces the size of global routing tables, which directly improves router performance and stability on the internet.