01. Private EC2 → NAT Gateway → IGW (IPv4 Flow)

Private EC2 → NAT Gateway → IGW (IPv4 Flow)

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Prerequisites

- EC2
- NAT Gateway
- IGW
- Subnet
- Route Table

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1. What we need

• Private EC2 (No Public IP)
• Private Route Table
• NAT Gateway (Located in Public Subnet)
• Public Route Table
• Internet Gateway (IGW)

2. Example

Step 1 — Request from Private EC2

Private EC2 (10.0.2.15) → External server (8.8.8.8)

Source: 10.0.2.15
Destination: 8.8.8.8

Step 2 — Private Route Table

10.0.0.0/16 → local
0.0.0.0/0 → NAT Gateway

👉 External traffic is routed to NAT Gateway

Step 3 — NAT Gateway (Core)

Performs IP translation (NAT)

Before:
Source: 10.0.2.15

After:
Source: 3.25.100.10 (Elastic IP)

Step 4 — Public Route Table

10.0.0.0/16 → local
0.0.0.0/0 → Internet Gateway

👉 Forwarded to IGW

Step 5 — Internet Gateway → Internet

Traffic is sent to external destination (8.8.8.8)

Step 6 — Response Flow

Internet (8.8.8.8)
→ IGW
→ NAT Gateway
→ Private EC2 (10.0.2.15)

3. Full Flow (One Line)

Private EC2 (10.0.2.15)
→ Private Route Table (0.0.0.0/0 → NAT)
→ NAT Gateway (translated to 3.25.100.10)
→ Public Route Table (0.0.0.0/0 → IGW)
→ Internet Gateway
→ Internet

4. Why NAT Gateway is Needed?

🔸 Maintain Private Subnet

• No Public IP required
• Improved security

🔸 Outbound Only

• Can send requests to Internet
• Cannot receive inbound connections

🔸 IP Masking

• Internal IP is hidden
• Only Elastic IP is visible externally