CIDR and Subnet Mask

Ganesh Pawar — Sun, 11 Oct 2026 09:29:33 +0000

Hello All,

I would like to share something I learned while preparing for the AWS Certified Advanced Networking - Specialty (ANS-C01) certification.

One of the topics that took me some time to fully understand was:

Decimal-to-binary conversion
How CIDR notation works
How to determine the network and broadcast address of a subnet

This may be a topic that others have already covered, but I thought it would still be helpful to share my learning journey.

Decimal to Binary

Computers do not understand decimal numbers like 192.168.31.1
Computers work with bits (0s and 1s), so the IP address must first be converted into binary before subnet calculations can be performed.
Rules:
If the remaining number is >= column value → write 1
If the remaining number is < column value → write 0

Each octet contains 8 bits.

The place values for an octet are: 128 64 32 16 8 4 2 1

Each position can be either:
1 = include the value
0 = do not include the value

Example : Convert 192 to Binary
Value Compare Bit Remaining
128 192 ≥ 128 1 64
64 64 ≥ 64 1 0
32 0 < 32 0 0
16 0 < 16 0 0
8 0 < 8 0 0
4 0 < 4 0 0
2 0 < 2 0 0
1 0 < 1 0 0
Result:11000000

CIDR: 192.168.31.1/18
Let's try to fetch IP range for above CIDR.

A /18 mask uses the first 18 bits for the network portion.
In the third octet, the mask is 192 (11000000 in binary).
This creates subnet blocks of 64 in the third octet:
0-63
64-127
128-191
192-255

Since the third octet is 31, it falls within the 0-63 block.

Conceptual Steps:

Convert IP address to a 32-bit binary number.

E.g. 192.168.31.1/18 Binary is 11000000.10101000.00011111.00000001

Create the subnet mask from the CIDR prefix. o For example, /18 means first 18 bits are 1, rest 14 bits are 0. 11111111.11111111.11000000.00000000
Perform a bitwise AND operation between the IP and the subnet mask.

Subnet mask in binary: 11111111.11111111.11000000.00000000
IP address in binary: 11000000.10101000.00011111.00000001
AND bitwise Operator results: 11000000.10101000.0000000.0000000
This zeroes out the host bits, leaving only the network bits.

The bitwise AND operator is used to compare two binary numbers bit by bit. For each bit, the result is 1 if both bits are 1, and 0 if at least one of the bits is 0.

Convert the result back to the dotted decimal format. o This is the network address. which is 11000000.10101000.0000000.0000000 (128+64).(128+32+8).0.0=192.168.0.0 for 192.168.31.1/18
Set Host Bits to 1 to Find the Broadcast Address: • The broadcast address is the last address in the subnet, which is obtained by setting all the host bits (bits corresponding to the 0s in the subnet mask) to 1. • Since /18 leaves the last 14 bits for hosts, set those 14 host bits to 1 in the network address: Network Address (Binary): 11000000.10101000.00000000.00000000 Set the host bits (the last 14 bits) to 1: 11000000.10101000.00111111.11111111 This binary value converts back to: 192.168.63.255

Therefore:

Network address: 192.168.0.0/18
Broadcast address: 192.168.63.255
Usable host range: 192.168.0.1 to 192.168.63.254

For AWS VPC networking, there is one important difference from traditional networking:

For the subnet 192.168.0.0/18:

Type IP AddressNetwork Address 192.168.0.0
Broadcast Address (traditional networks) 192.168.63.255
Traditional Usable Range 192.168.0.1 - 192.168.63.254

However, in AWS VPCs, AWS reserves 5 IP addresses per subnet:

Reserved Address Purpose
192.168.0.0 Network address
192.168.0.1 VPC router
192.168.0.2 Amazon DNS
192.168.0.3 Reserved by AWS
192.168.63.255 Network broadcast address (reserved by AWS, even though AWS does not support broadcast traffic)

DEV Community: Ganesh Pawar

CIDR and Subnet Mask