Subnet Mask Binary Calculator

What Is a Subnet Mask?

A subnet mask is a 32-bit IPv4 value used to distinguish the network portion of an IPv4 address from the remaining host portion. In binary form, a valid subnet mask begins with a continuous sequence of 1 bits and ends with a continuous sequence of 0 bits.

For example, the common subnet mask 255.255.255.0 contains twenty-four leading 1 bits and eight trailing 0 bits. It can therefore also be written using CIDR notation as /24.

How the Subnet Mask Binary Calculator Works

This calculator accepts a CIDR prefix, dotted-decimal subnet mask or 32-bit binary subnet mask and converts it into the other standard representations.

The tool also checks whether a decimal or binary mask is valid. A correct IPv4 subnet mask cannot contain a 1 after the first 0 has appeared.

CIDR Prefix to Subnet Mask

CIDR notation expresses the number of leading network bits using a slash followed by a number from 0 through 32.

The first 24 mask bits are set to 1 and the remaining 8 are set to 0:

Converting those four binary octets to decimal gives:

Subnet Mask to CIDR Prefix

To convert a valid subnet mask to CIDR notation, count the number of 1 bits in its binary representation.

The remaining six bits are the host portion of the mask.

Subnet Mask Binary Values

Because valid subnet-mask bits must remain contiguous, an individual subnet-mask octet has only a small number of valid decimal values.

Common Subnet Masks in Binary

Why Subnet Masks Use Contiguous 1 Bits

CIDR subnet masks are defined by a prefix length. A prefix represents a continuous block of network bits beginning at the most-significant side of the address.

For that reason, a valid mask cannot switch from 1 to 0 and then back to 1 later in the bit pattern.

The calculator checks this rule when converting a user-entered decimal or binary mask.

Subnet Mask Octet Example

Consider the mask:

Its binary octets are:

Counting the set bits gives:

Therefore the CIDR prefix is:

What Are Network Bits and Host Bits?

The CIDR prefix length indicates how many of the 32 IPv4 mask positions belong to the network prefix. The remaining positions are host bits.

This calculator reports these bit counts but intentionally does not use them to calculate host capacity or subnet address ranges.

/24 Subnet Mask in Binary

A /24 subnet mask is one of the most commonly encountered IPv4 masks.

The first three complete octets are network bits, while the fourth octet contains eight host-bit positions.

/26 Subnet Mask in Binary

A /26 mask contains twenty-six network bits.

The last octet 11000000 contains two network bits followed by six host bits.

/20 Subnet Mask Example

A /20 prefix crosses the third IPv4 octet instead of ending exactly on an octet boundary.

This example demonstrates why knowing the binary subnet-mask pattern can be useful when learning or troubleshooting IPv4 networking.

Subnet Mask vs IPv4 Address

An IPv4 address and subnet mask are both 32-bit values, but they serve different purposes.

That is why BinaryCon keeps the IPv4 Binary Calculator and Subnet Mask Binary Calculator as separate tools.

Subnet Mask vs CIDR Notation

CIDR notation and dotted-decimal subnet masks are two representations of the same prefix information.

CIDR is compact, while dotted-decimal notation makes the four IPv4 octets visible.

Subnet Mask Hexadecimal Representation

A subnet mask can also be displayed as four hexadecimal bytes. Each decimal octet maps directly to two hexadecimal digits.

For a /26 mask:

Hexadecimal representation can be useful when inspecting low-level networking data or packet structures.

Why Binary Subnet Masks Matter

Computers evaluate IPv4 addresses and masks at the binary level. Viewing the mask as 32 individual bits makes the relationship between CIDR prefixes and dotted-decimal notation easier to understand.

Binary masks are useful for networking education, certification study, router and firewall configuration review, packet analysis, troubleshooting and software development involving IP address processing.

Subnet Mask Validation

Not every four-octet value from 0 through 255 is a valid subnet mask. The bits must form one uninterrupted sequence of 1s followed by one uninterrupted sequence of 0s.

For example:

The decimal value 253 is binary 11111101, which contains a 0 followed later by another 1. That breaks the contiguous-prefix rule.

Important Subnet Mask Notes

Subnet Mask Binary Calculator FAQs