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05. Subnet

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05. Subnet

Subnet

Prerequisites

1. Subnet

"aws-vpc0"

Subnet is a core component of VPC. It provides Logical segmentation of a VPC network

A Subnet is:

  • A smaller network inside a VPC
  • Defined by a portion of the VPC IP range
  • Used to organize and isolate resources

👉 Think of it as:

Dividing one big network into smaller, controlled networks

Each subnet has its own CIDR block.

"aws-subnet5"

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VPC:    10.0.0.0/16
Subnet: 10.0.1.0/24
Subnet: 10.0.2.0/24

Each subnet belongs to Exactly one Availability Zone (AZ)

2. Why Subnet works

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Region
 └── VPC
      ├── Subnet A (AZ-a)
      └── Subnet B (AZ-b)
  • High availability design
  • Security/Fault isolation
  • Multi-AZ architecture
  • Network segmentation
  • Traffic control

2-1. Public Subnet

  • Has route to Internet Gateway (IGW) and Nat Gateway
  • Can communicate with the internet

👉 Used for:

  • Web servers
  • Load balancers

2-2. Private Subnet

  • No direct route to IGW
  • Not accessible from the internet

👉 Used for:

  • Databases (RDS)
  • Internal services

2-3. Subnet Design Pattern

Subnet itself does not enforce strong security.

Security is handled by:

  • Security Group (instance-level)
  • NACL (subnet-level)

Typical architecture:

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Internet
   ↓
[ Public Subnet ]
   └── Load Balancer / Bastion
   ↓
[ Private Subnet ]
   └── Application / Database

Example:

  • EC2 → choose subnet
  • RDS → placed in private subnet
  • ELB → placed in public subnet

3. How to create Subnet

3-1. Search VPC

"aws-vpc0"

3-2. Click Navigation pane → “Subnets”

"aws-subnet1"

3-3. Click Button → “Create Subnet”

"aws-subnet2"

3-4. Step 1. Select VPC & Create public Subnet

"aws-subnet3"

3-4. Step 2. Create private Subnet

"aws-subnet4"

  • Components
    • VPC
    • Internet Gateway
    • Router table
    • Nat Gateway
AWS, AWS - Fundamental
AWS AWS - Fundamental
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