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IP/24SubnetLens

CIDR calculator

Drag the prefix and watch the address space halve. The bit boundary, the host count and the block all move together.

Try
10.0.0.0/27
Entered10.0.0.15
30usable addresses
First usable
10.0.0.1
Last usable
10.0.0.30
Subnet 10 dot 0 dot 0 dot 0 slash 27. 32 total addresses, 30 usable host addresses. First usable 10 dot 0 dot 0 dot 1, last usable 10 dot 0 dot 0 dot 30. Broadcast 10 dot 0 dot 0 dot 31. The entered address 10 dot 0 dot 0 dot 15 sits at offset 15 inside the block.
Usable hosts32 total
30
Common
Usable hosts
30
32 total
Bits
27 network/5 host

The prefix is a boundary, not a size

A /24 does not mean “256 addresses” by decree. It means the first 24 bits of every address in the block are identical, leaving 8 bits free — and 8 free bits is 2⁸ = 256 addresses. Move the boundary one bit right and you halve the block; one bit left and you double it. Open Details to watch the boundary slide along the 32-bit strip as you drag.

Splitting and merging

Split in half divides the current block into two of the next size down and keeps the one containing your address lit; the sibling stays visible so you can see what you gave up. Click a dimmed sibling to move into it. One level up merges back into the parent block. This is subnetting and supernetting, without the arithmetic.