We can find Subnet Mask from any below information available.

How to find Subnet Mask from prefix?

Prefix or CIDR Netmask represents network part.

So, Network Bits = Prefix #

Subnet Mask is 32 bits binary number. Which consists of Network Bits and Host Bits.SM = 32 and NB = prefix#

SM = NB + HB

SM = prefix# + HB

To find Subnet Mask form32 bit binary number which has consicutive prefix# times '1' and remaining bits as 0

Divide this binary number in 4 octets and convert octets to decimal format.

Example:

Subnet Mask for /23 network

SM = 11111111111111111111111

Now append consecutive '0' HB times with SM. here HB = 9.

SM = 11111111111111111111111000000000

Divide in 4 octets

SM = 11111111.11111111.11111110.00000000

Convert octets to decimal

SM = 255.255.254.0

How to find Subnet Mask from Network Bits or Host Bits?

As mentioned before Subnet Mask is 32 bits binary number. Which consists of Network Bits and Host Bits.

SM = NB + HB

To find Subnet Mask form32 bit binary number which has consicutive prefix# times '1' and remaining bits as 0

Divide this binary number in 4 octets and convert octets to decimal format.

Example:

Subnet Mask for Network Bits 25 or Host Bits 7

SM = 1111111111111111111111111

Now append consecutive '0' HB times with SM. here HB = 7.

SM = 11111111111111111111111110000000

Divide in 4 octets

SM = 11111111.11111111.11111111.10000000

Convert octets to decimal

SM = 255.255.255.128

How to find Subnet Mask from Wildcard Mask?

Subnet Mask is inverted number of Wildcard Mask.

SM = ~ WM

Example:

Subnet Mask for Wilcard Mask 0.0.0.127

WM = 00000000.00000000.00000000.01111111

Subnet Mask is inverted binary of Wildcard Mask.

SM = 11111111.11111111.11111111.10000000

Convert octets to decimal

SM = 255.255.255.128

How to find Subnet Mask from Class information?

Classful IP address allocating scheme were designed on basic of predefined block of IP ranges.

Default Subnet Mask for classes are as below

Class | Subnet Mask |
---|---|

A | 255.0.0.0 |

B | 255.255.0.0 |

C | 255.255.255.0 |

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