Subnet Calculator
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Work in the notation network equipment actually uses: an address and a dotted-decimal mask. The calculator reports the network, the broadcast, the usable range and the host count — and if you give it a longer prefix, it splits the block into equal subnets and lists them, which is the half of subnetting that a CIDR lookup does not answer.
How to use Subnet Calculator
- 1
Enter the IP address and pick the subnet mask.
- 2
To split the block, set a new prefix longer than the current one.
- 3
Read the resulting subnets, each with its own address count.
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Frequently asked questions
How do I split a network into smaller subnets?+
Borrow bits from the host portion. Going from /24 to /26 borrows two bits, which gives four subnets of 64 addresses each — 62 usable, since each keeps its own network and broadcast address. Every bit borrowed doubles the number of subnets and halves their size.
How many hosts fit in a /26?+
62. A /26 has 64 addresses; the first is the network address and the last is the broadcast, and neither can be assigned to a machine. That pair is why the usable count is always two below the power of two — except on a /31 or /32, where the rule does not apply.
Why does splitting reduce the total usable hosts?+
Because each new subnet loses its own network and broadcast address. One /24 has 254 usable hosts; four /26s have 248 between them. Six addresses vanish into overhead — which is the real cost of subnetting more finely than you need to.
What is the difference between a mask and a prefix?+
Nothing but notation. 255.255.255.0 and /24 are the same 24 leading one-bits. Routers and older documentation use the dotted form, CIDR notation uses the count, and this tool accepts either.
Can a subnet mask be something like 255.0.255.0?+
No. The one-bits have to be contiguous from the left — that has been a requirement since CIDR replaced classful addressing. A discontiguous mask is a valid-looking address and an invalid mask, and this tool rejects it rather than computing something meaningless from it.