CIDR to Subnet Mask Converter
Convert any IPv4 CIDR prefix into its dotted-decimal subnet mask. Move the prefix control and the matching mask, wildcard mask, address count and network/host boundary update instantly.
How CIDR converts to a subnet mask
Each CIDR prefix represents a run of one-bits followed by zero-bits in a 32-bit subnet mask. /24 is twenty-four ones followed by eight zeros, which converts to 255.255.255.0.
The conversion is deterministic: /25 is 255.255.255.128, /26 is 255.255.255.192, /27 is 255.255.255.224, /28 is 255.255.255.240 and /30 is 255.255.255.252.
Fast conversion method
Every full 8 network bits produces an octet of 255. The remaining partial octet can be read from the sequence 0, 128, 192, 224, 240, 248, 252, 254, 255. For example, /20 has two full octets plus four network bits in the third octet, producing 255.255.240.0.
CIDR and address capacity
Converting the prefix also reveals the number of host bits. A /24 leaves 8 host bits, /28 leaves 4 and /30 leaves 2. Total addresses are calculated as 2 raised to the number of host bits.
Use the full prefix reference
For frequent network planning, the subnetting cheat sheet lists every IPv4 prefix alongside its subnet mask, wildcard mask, total address count and conventional usable-host count.
What this calculator shows
The result panel reports the canonical network address, broadcast address, first and last conventional usable addresses, dotted-decimal subnet mask, wildcard mask, total IPv4 address count and usable-host count. The live 32-bit boundary updates with the CIDR prefix, so the relationship between prefix length, block size and address capacity is visible instead of hidden behind a static table.
How to read the result correctly
The calculator normalizes the IP address you enter to the network block that contains it. That means an input such as 192.168.1.42/24 correctly resolves to network 192.168.1.0/24. For prefixes /0 through /30, conventional IPv4 subnetting reserves the first address as the network identifier and the final address as broadcast. /31 and /32 have different practical semantics, so their address counts should not be interpreted with the ordinary “subtract two” rule.
Related subnet tools
You can also use the subnetting cheat sheet for a complete IPv4 prefix reference, browse individual CIDR prefix pages, try the IPv6 subnet calculator, or return to the main visual subnet calculator.
Frequently asked questions
What subnet mask is /24?
/24 is 255.255.255.0.
What subnet mask is /30?
/30 is 255.255.255.252 and contains 4 total IPv4 addresses.
Can every subnet mask be written as CIDR?
Every conventional contiguous subnet mask can be represented as CIDR. Non-contiguous masks do not have a standard CIDR prefix equivalent.