How to find Subnet Mask?

We can find Subnet Mask from any below information available.

Prefix (CIDR Netmask)
Network Bits
Host Bits
Wildcard Mask
Class information

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 form 32 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

Write 23 times consecutive '1' as Prefix# = 23
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

Hence for network /23 Subnet Mask is 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 form 32 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

Write 25 times consecutive '1' as NB = 25
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

Hence for network /25 Subnet Mask is 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

Write Wilcard Mask in binary
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

Hence for Wilcard Mask 0.0.0.127 Subnet Mask is 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