Subnet calculator
What that slash actually covers.
Examples temporarily replace the input and settings. You can restore your previous work.
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Output
Paste or type above to see the result here.
Your files stay on your device
This tool runs entirely in your browser. What you paste is processed on your device and is never sent to a server, so it is safe to use with tokens, keys and unpublished text.
Example
Example: an office /24 split into four /26 subnets
The office network is 10.20.0.0/24 and needs four separate subnets: staff, guests, printers and cameras. Type 10.20.0.0/24 and choose “Also split it into” /26 blocks.
- First the whole /24: network 10.20.0.0, broadcast 10.20.0.255, hosts 10.20.0.1 – 10.20.0.254, 254 usable, private (RFC 1918).
- Then “Split into /26: 4 blocks”: 10.20.0.0/26, 10.20.0.64/26, 10.20.0.128/26 and 10.20.0.192/26, 62 usable hosts each.
- Which block is a device in? 10.20.0.77/26 gives network 10.20.0.64, broadcast 10.20.0.127 and hosts 10.20.0.65 – 10.20.0.126.
| Prefix | Netmask | Usable hosts | Blocks in a /24 |
|---|---|---|---|
| /24 | 255.255.255.0 | 254 | 1 |
| /25 | 255.255.255.128 | 126 | 2 |
| /26 | 255.255.255.192 | 62 | 4 |
| /27 | 255.255.255.224 | 30 | 8 |
| /28 | 255.255.255.240 | 14 | 16 |
| /29 | 255.255.255.248 | 6 | 32 |
| /30 | 255.255.255.252 | 2 | 64 |
Four /26 subnets with the mask 255.255.255.192 and 62 hosts each; the guests’ one, 10.20.0.64/26, hands out 10.20.0.65 to 10.20.0.126.
How it works
- Type an address with a prefix (192.168.1.10/24), with a mask (192.168.1.10 255.255.255.0), or on its own.
- Read off the network, the broadcast and the range you can actually assign.
- One address per line if you have several.
- To carve it up, choose a smaller prefix to split into.
Frequently asked questions
- Why are there two fewer usable hosts than addresses?
- The first address in a block names the network and the last is the broadcast, so neither can be given to a machine. A /24 has 256 addresses and 254 usable ones.
- What about a /31?
- It is the exception, and it is called out. A /31 is a point-to-point link: both addresses are usable and there is no broadcast, which is why it is used between routers. A /32 is a single host.
- Will it take a mask like 255.0.255.0?
- No, and it says so. A mask has to be a run of ones followed by a run of zeros; anything else is not a subnet mask, however plausible it looks.
- Does it do IPv6?
- Not yet. IPv6 subnetting is mostly a matter of counting nibbles and rarely needs a calculator in the same way, so this is IPv4.
- What is the subnet mask for /26?
- 255.255.255.192: 26 ones and then 6 zeros, so 64 addresses and 62 usable hosts. A /25 is 255.255.255.128 (126 hosts), a /27 255.255.255.224 (30) and a /28 255.255.255.240 (14).
- How many /26 subnets fit in a /24?
- Four. Each step in the prefix halves the block, so a /24 holds 2 /25s, 4 /26s, 8 /27s and so on, down to 64 /30s. Choose the size under “Also split it into” and every block is listed.
- Is anything sent to a server?
- No. It runs in your browser, so what you paste stays on your computer.
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