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

Subnet Mask Validator

Check mask structure and contiguity.

Subnet Mask Validator: 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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Wildcard Mask to Subnet Mask Converter

Convert compatible contiguous wildcard masks.

ACL Wildcard Match Calculator

Evaluate address-and-wildcard matching.

How Subnet Mask Validator Works?

The Subnet Mask Validator works by checking 3 properties of a mask: 4 octets, every octet from 0 to 255, and all 1 bits placed before all 0 bits.

  1. Enter a mask

    Type a value such as 255.255.255.250.

  2. Check the format

    The mask must have exactly 4 decimal octets.

  3. Check the octet range

    Every octet must be between 0 and 255.

  4. Check contiguity

    The validator inverts the mask and adds 1. A valid mask produces an exponent of 2. 255.255.255.250 inverts to 5, and 5 + 1 = 6 fails.

Subnet Mask Validator Formula

The Subnet Mask Validator formula is Valid = ((NOT Mask) AND ((NOT Mask) + 1)) = 0, which is true only for contiguous masks.

Inverse

I = NOT M = 2^32 − 1 − M

The wildcard as an integer.

Contiguity test

(I AND (I + 1)) = 0

True when I is a run of 1 bits on the right.

Valid octet values

{0, 128, 192, 224, 240, 248, 252, 254, 255}

Any other octet value fails.

Worked example: 255.255.255.250

250 = 11111010
Inverse last octet = 00000101 = 5
5 AND 6 = 00000101 AND 00000110 = 4 ≠ 0
Result = Invalid (non-contiguous 1 bits)

Subnet Mask Validator Conversion Table

The Subnet Mask Validator conversion table checks 12 sample masks and shows the result and reason or prefix for each.

MaskBinaryResultReason / Prefix
255.255.255.011111111.11111111.11111111.00000000Valid/24
255.255.0.25511111111.11111111.00000000.11111111InvalidNon-contiguous 1 bits
255.255.255.25011111111.11111111.11111111.11111010InvalidNon-contiguous 1 bits
255.255.255.256—InvalidOctet above 255
0.0.0.000000000.00000000.00000000.00000000Valid/0
255.255.255.25511111111.11111111.11111111.11111111Valid/32
255.0.255.011111111.00000000.11111111.00000000InvalidNon-contiguous 1 bits
255.255.128.011111111.11111111.10000000.00000000Valid/17
255.255.192.111111111.11111111.11000000.00000001InvalidNon-contiguous 1 bits
255.254.0.011111111.11111110.00000000.00000000Valid/15
255.255.255.25311111111.11111111.11111111.11111101InvalidNon-contiguous 1 bits
255.255.255—InvalidNot 4 decimal octets

What is Subnet Mask Validator?

The Subnet Mask Validator is a tool that checks whether a dotted-decimal value is a valid IPv4 subnet mask. A valid mask has 1 bits on the left and 0 bits on the right with no gaps.

RFC 950 permitted non-contiguous masks, but RFC 4632 and every modern implementation require contiguous masks. Devices reject masks such as 255.0.255.0, or worse, accept them and route unpredictably.

Typos are the main source of invalid masks: 255.255.255.250 instead of 255.255.255.252, or 255.255.225.0 instead of 255.255.224.0. The validator catches each one with a clear reason.

What are the uses of Subnet Mask Validator?

There are 4 main uses of the Subnet Mask Validator, including form validation, config audits, and typo detection.

  • Validating form fields

    Web forms for network settings reject invalid masks before saving.

  • Auditing device configurations

    Audit scripts flag interfaces with non-contiguous or mistyped masks.

  • Catching typos in tickets

    Change tickets with 255.255.225.0 fail validation before implementation.

  • Teaching mask structure

    Students learn why only 9 octet values appear in valid masks.