Convert between CIDR notation and subnet masks with network range and host count calculations.
CIDR notation packs a network address and prefix length into one string, like 192.168.1.0/24, and this calculator expands that shorthand into the numbers you actually need: the dotted-decimal subnet mask, the network and broadcast addresses, the first and last usable host, and the total address count for the block. Enter either a CIDR string or an address paired with a mask and it converts between the two. The detail people trip over most: type a host address that isn't already the start of its block and the tool rounds down to the enclosing network rather than treating your address as special.
A concrete input and expected output from the current implementation.
Input
10.20.30.40/27
Expected output
Network address: 10.20.30.32, Broadcast address: 10.20.30.63, Subnet mask: 255.255.255.224, Usable host range: 10.20.30.33 - 10.20.30.62, Total addresses: 32, Usable hosts: 30
A /27 leaves 5 host bits, so each block spans 32 addresses starting on multiples of 32; 40 falls between 32 and 63, so the tool reports that block's network (.32) and broadcast (.63), leaving 30 usable addresses in between.
The prefix length (the number after the slash) sets how many leading bits are network bits; the tool converts that count directly into a 32-bit subnet mask and its dotted-decimal form, and can also work backward, taking a mask like 255.255.255.224 and reporting the equivalent /27 prefix.
It ANDs the entered address with the mask to find the network address, sets the host bits to all 1s for the broadcast address, and reports the addresses in between as the usable host range. Total addresses are 2^(32-prefix); usable hosts are normally that count minus 2 for the network and broadcast addresses.
No. A /27 network starts every 32 addresses (0, 32, 64, 96...), so any address in that block is rounded down to its network boundary. 10.20.30.40 falls between .32 and .63, so .32 is the correct network address for that /27.
For ordinary subnets (prefix /30 and shorter) yes, it subtracts both. /31 and /32 are special cases under RFC 3021 and single-host routing where that subtraction doesn't apply the normal way, so double-check the output for those two prefixes specifically rather than assuming minus-2 always holds.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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