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IP Range CalculatorPRO
Masks, Wildcards & ACLInstant Client-Side Tool

Subnet Mask to CIDR Converter

Find prefix length from contiguous masks.

Subnet Mask to CIDR Converter: inputs and results

Enter an IP address, CIDR prefix, or range. Then check its first and last address below.

Quick presets:

Calculated Boundaries & ParametersReady

Network Address192.168.1.0
Broadcast Address192.168.1.255
First Usable Host192.168.1.1
Last Usable Host192.168.1.254
Subnet Mask255.255.255.0
Wildcard Mask0.0.0.255
Total Addresses256
Usable Host Range254

Key Capabilities

  • •100% Client-Side: Zero telemetry or server latency.
  • •RFC Compliant: Exact 32-bit and 128-bit integer precision.
  • •Quick Copy: Instant access to CIDR, mask, and range bounds.
  • •Cross-Platform: Works on desktop, tablet, and mobile browsers.
Open Full Interactive Suite

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How Subnet Mask to CIDR Converter Works?

This tool works by confirming that the mask is contiguous and then counting its 1 bits. The count is the CIDR prefix length.

  1. Enter a dotted mask

    Type a mask such as 255.255.252.0.

  2. Check each octet value

    Every octet must be one of 9 values: 0, 128, 192, 224, 240, 248, 252, 254, or 255.

  3. Check contiguity

    An octet below 255 must be followed only by 0 octets.

  4. Count the 1 bits

    255 (8) + 255 (8) + 252 (6) + 0 (0) = 22 → /22.

Subnet Mask to CIDR Converter Formula

The formula is p = 32 − log2(2^32 − Mask), which equals the number of 1 bits in a contiguous mask.

Bit count

p = Σ popcount(octet)

Counts the 1 bits.

Integer form

p = 32 − log2(2^32 − M)

M is the mask as an integer.

Octet lookup

128→1, 192→2, 224→3, 240→4, 248→5, 252→6, 254→7, 255→8

Bits contributed by each octet value.

Worked example: 255.255.252.0

255 → 8 bits
255 → 8 bits
252 → 11111100 → 6 bits
0   → 0 bits
CIDR = /22

Subnet Mask to CIDR Converter Conversion Table

The table maps 16 dotted masks, where the 3rd or 4th octet holds each of the 8 partial values, to their CIDR prefix.

Subnet MaskPartial OctetPartial Octet BinaryCIDR
255.255.128.012810000000/17
255.255.192.019211000000/18
255.255.224.022411100000/19
255.255.240.024011110000/20
255.255.248.024811111000/21
255.255.252.025211111100/22
255.255.254.025411111110/23
255.255.255.025511111111/24
255.255.255.12812810000000/25
255.255.255.19219211000000/26
255.255.255.22422411100000/27
255.255.255.24024011110000/28
255.255.255.24824811111000/29
255.255.255.25225211111100/30
255.255.255.25425411111110/31
255.255.255.25525511111111/32

What is Subnet Mask to CIDR Converter?

The Subnet Mask to CIDR Converter is a tool that turns a dotted-decimal IPv4 subnet mask into its CIDR prefix length. 255.255.252.0 becomes /22.

Prefix notation is shorter and fits directly after an address, as in 10.0.0.0/22. Cloud platforms, Linux ip commands, and routing tables use it everywhere. The converter turns masks copied from older devices into that format.

The converter accepts contiguous masks only. Non-contiguous masks such as 255.255.0.255 have no CIDR equivalent, and the converter flags them as invalid.

What are the uses of Subnet Mask to CIDR Converter?

There are 4 main uses of the Subnet Mask to CIDR Converter, including migrations, cloud entries, and log analysis.

  • Migrating firewall objects

    Objects stored with dotted masks become CIDR entries on the new platform.

  • Entering cloud network ranges

    AWS, Azure, and Google Cloud consoles accept CIDR only.

  • Reading Linux route tables

    Comparing route -n mask output with ip route CIDR output requires conversion.

  • Shortening documentation

    Design tables use /22 instead of 255.255.252.0 to save space.