CIDR Calculator
Type a CIDR block and get the full breakdown instantly — plus a one-click way to split it into equal subnets.
CIDR Calculator
Breakdown
Split This Block
Break this CIDR block into equal-sized subnets, either by count or by target prefix.
| # | Network | Broadcast | |
|---|---|---|---|
| 1 | 10.0.0.0/24 | 10.0.0.255 | |
| 2 | 10.0.1.0/24 | 10.0.1.255 | |
| 3 | 10.0.2.0/24 | 10.0.2.255 | |
| 4 | 10.0.3.0/24 | 10.0.3.255 |
Have an IP and mask instead of a CIDR block? Try the general subnet calculator · Need just the address range? See CIDR to IP range · Combining several blocks into one? Use the supernet calculator
What CIDR notation is
CIDR (Classless Inter-Domain Routing) notation writes a network as an address followed by a slash and a
number, like 10.0.0.0/22.
The number after the slash is the prefix length: how many of the address's 32 bits are fixed as the
network portion. The remaining bits belong to hosts within that network. A larger prefix number means
fewer host bits and therefore a smaller block — a /28 is much smaller than a /16, even though 28 is the
bigger number.
This is the same information a dotted-decimal subnet mask carries, just expressed as a bit count instead
of four octets. /24 and
255.255.255.0 describe
the identical boundary. If you're starting from a mask or a wildcard mask instead of a CIDR block, the
subnet mask calculator
converts between all three formats.
Prefix length to block size
Every prefix length corresponds to a fixed number of addresses: 2^(32 − prefix).
Stepping the prefix down by one bit always doubles the block size; stepping it up by one bit always
halves it. That's the entire arithmetic behind converting between prefix lengths.
| Prefix | Subnet Mask | Total Addresses | Usable Hosts |
|---|---|---|---|
| /16 | 255.255.0.0 | 65,536 | 65,534 |
| /17 | 255.255.128.0 | 32,768 | 32,766 |
| /18 | 255.255.192.0 | 16,384 | 16,382 |
| /19 | 255.255.224.0 | 8,192 | 8,190 |
| /20 | 255.255.240.0 | 4,096 | 4,094 |
| /21 | 255.255.248.0 | 2,048 | 2,046 |
| /22 | 255.255.252.0 | 1,024 | 1,022 |
| /23 | 255.255.254.0 | 512 | 510 |
| /24 | 255.255.255.0 | 256 | 254 |
| /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 |
How splitting a block works
Splitting a CIDR block means carving it into smaller, equal-sized blocks by extending the prefix. Each extra bit of prefix doubles the number of resulting blocks and halves each one's size. A /20 extended by 2 bits becomes four /22s; extended by 4 bits it becomes sixteen /24s. The tool above accepts either target — "I need N subnets" or "I want /24 blocks" — and computes the same result either way, since the two are directly related: the number of blocks is always a power of two.
This matters when planning VLANs, cloud subnets, or point-to-point links from a single allocation: you get the exact list of resulting network addresses without manually adding block sizes in binary. Working the other direction — combining several existing blocks back into one covering range — is what the supernet calculator does, and allocating subnets of genuinely different sizes from one block (rather than equal splits) is what the VLSM calculator handles.
CIDR Calculator FAQ
How many /24 subnets fit in a /16?
256. Each step down in prefix length by one bit doubles the number of blocks, so going from /16 to /24 is 8 extra bits — 2^8 = 256 /24 blocks. The same doubling rule applies between any two prefix lengths: a /20 splits into 16 /24s, and a /22 splits into 4 /24s.
What does the /24 in CIDR mean?
The /24 is the prefix length: it says the first 24 of the 32 bits in the IPv4 address are fixed as the network portion, leaving 8 bits (256 addresses) for hosts. It's shorthand for the subnet mask 255.255.255.0 — both describe the exact same boundary between network and host bits.
How do I split a CIDR block into equal subnets?
Decide either how many subnets you need or how large each one should be, then extend the prefix by enough bits to cover it. Splitting a /20 into 4 equal pieces means adding 2 bits (2^2 = 4), giving four /22 blocks. This calculator's split feature does that arithmetic for you and lists every resulting block.
Is CIDR the same as a subnet mask?
They express the same information in different formats. A CIDR prefix like /24 is a bit count; a subnet mask like 255.255.255.0 is the dotted-decimal form of that same bit count. Converting between them is one of the outputs this calculator shows automatically for whatever block you enter.
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