How do you calculate for the decimal subnet

Question

ScanSolve AI Tutor · Accepted Answer

To calculate a decimal subnet mask from a CIDR prefix length (e.g., $/24$ ), follow these steps:

Understand the prefix length.
The prefix length (e.g., $N$ in $/N$ ) indicates the number of network bits from the left in a 32-bit IPv4 address. The remaining $32 - N$ bits are host bits.
Write the subnet mask in binary.
Create a 32-bit binary number. The first $N$ bits will be $1$ s, and the remaining $32 - N$ bits will be $0$ s.
Divide the binary mask into four 8-bit octets.
Group the 32 binary bits into four sets of 8 bits.
Convert each binary octet to its decimal equivalent.
Convert each 8-bit binary number into its decimal value. These four decimal values, separated by dots, form the dotted-decimal subnet mask.

Example: Calculate the decimal subnet mask for $/26$ .
The prefix length is $N=26$ .
This means there are 26 network bits and $32 - 26 = 6$ host bits.
Write the 32-bit binary mask
$\underbrace{11111111}_{8 \text{ bits}} \underbrace{11111111}_{8 \text{ bits}} \underbrace{11111111}_{8 \text{ bits}} \underbrace{11000000}_{8 \text{ bits}}$
(26 ones followed by 6 zeros)
Divide into four octets
- First octet: 11111111
- Second octet: 11111111
- Third octet: 11111111
- Fourth octet: 11000000
Convert each octet to decimal
- $11111111_2 = 255_{10}$
- $11111111_2 = 255_{10}$
- $11111111_2 = 255_{10}$
- $11000000_2 = (1 \times 128) + (1 \times 64) + (0 \times 32) + (0 \times 16) + (0 \times 8) + (0 \times 4) + (0 \times 2) + (0 \times 1) = 128 + 64 = 192_{10}$
The decimal subnet mask for $/26$ is $\boxed{255.255.255.192}$ .

Answer