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IP Addresses, Subnets & CIDR Explained

Learn how IP addresses are organized into networks and subnets, understand CIDR notation, and see why concepts like 10.0.0.0/16 are important when designing an AWS VPC.

Now let's answer an important question:
How are IP addresses organized into networks?
This is where subnets and CIDR become important.

1. What Is a Network?

Consider these IP addresses:
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192.168.1.10
192.168.1.20
192.168.1.30
They may belong to the same local network.
A network allows devices to communicate with each other according to the network's addressing and routing configuration.

2. What Is a Subnet?

A subnet is a smaller logical network created from a larger IP network.
Think of it like this:
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Large Network
│
┌────┼────┐
▼ ▼ ▼
Subnet Subnet Subnet
In AWS, we create subnets inside a VPC.
For example:
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VPC
└── 10.0.0.0/16
├── Subnet A
├── Subnet B
└── Subnet C
Subnets help us organize resources and control network traffic.

3. What Is CIDR?

CIDR stands for Classless Inter-Domain Routing.
It represents an IP network using:
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IP address / Prefix length
For example:
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10.0.0.0/16
Here:
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10.0.0.0 → Network address
/16 → Prefix length
The /16 tells us how many bits belong to the network portion of the address.

4. Understanding /24

Consider:
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192.168.1.0/24
A /24 IPv4 network contains 256 total addresses.
The range is:
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192.168.1.0
↓
192.168.1.255
So:
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192.168.1.0/24
represents a network containing 256 IPv4 addresses.

5. /16 vs /24

Compare:
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10.0.0.0/16
and:
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10.0.0.0/24
The /16 network is larger.
The /24 network is smaller.
A useful rule is:
Larger prefix number → smaller network
For example:
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/16 → larger network
/20 → smaller
/24 → smaller still

6. Why Does This Matter in AWS?

When creating an Amazon VPC, you choose an IP address range.
For example:
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VPC
10.0.0.0/16
You can then divide it into smaller subnets:
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VPC: 10.0.0.0/16

├── 10.0.1.0/24
├── 10.0.2.0/24
├── 10.0.3.0/24
└── 10.0.4.0/24
This gives us a structured way to organize AWS resources.
For example:
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Public Subnet
↓
Load Balancer

Private Subnet
↓
Application Server

Private Subnet
↓
Database
We will learn what public and private subnets actually mean in a later article.

7. A Simple Mental Model

Think of:
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7
VPC
↓
Network
↓
Subnets
↓
Resources
For example:
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10.0.0.0/16
↓
┌────┴────┐
▼ ▼
Public Private
Subnet Subnet
The subnet determines the network range in which resources receive their private IP addresses.
# aws-networking# networking
Any opinions in this article are those of the individual author and may not reflect the opinions of AWS.
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