# AWS Private IP vs Public IP vs Elastic IP

When you start working with AWS - especially with EC2 instances - one of the most confusing topics is IP addressing. You launch a server, and suddenly you see **Private IP**, **Public IP**, and sometimes something called an **Elastic IP**.

Understanding the difference between these three is critical if you’re preparing for AWS certifications, working in DevOps, or building production infrastructure.

Private IP is used for internal communication inside a VPC, Public IP allows internet access, and Elastic IP is a static public IP you control and can remap.

Let’s break everything down simply and practically.

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## 1️⃣ What is a Private IP in AWS?

A **Private IP address** is assigned to an EC2 instance within a VPC (Virtual Private Cloud). It is used for **internal communication** between resources inside the AWS network.

Private IPs:

* Are assigned from your VPC CIDR block (e.g., 10.0.0.0/16)
    
* Cannot be accessed directly from the internet
    
* Remain with the instance for its lifetime
    
* Are used for backend communication (e.g., app server → database)
    

### Example

If you launch two EC2 instances in the same VPC:

* Instance A: 10.0.1.10
    
* Instance B: 10.0.1.20
    

They can communicate using these private IPs without going over the internet.

### When to Use Private IP

* Connecting application servers to databases
    
* Internal microservices communication
    
* Backend-only systems
    
* Secure internal networking
    

In real-world production setups, databases like RDS are accessed only via private IPs for security.

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## 2️⃣ What is a Public IP in AWS?

A **Public IP address** allows your EC2 instance to communicate with the internet.

Public IPs:

* Are assigned automatically (if enabled)
    
* Change when you stop and start the instance
    
* Allow inbound/outbound internet traffic
    
* Are mapped to the instance’s private IP
    

If your EC2 instance is in a public subnet and has an Internet Gateway attached to the VPC, it can receive a public IP.

### Example

You launch a web server:

* Private IP: 10.0.1.15
    
* Public IP: 3.110.45.123
    

Users access your website via the public IP.

### Important Limitation

If you:

* Stop the instance
    
* Start it again
    

The public IP changes.

This is a big problem for production systems.

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## 3️⃣ What is an Elastic IP (EIP)?

An **Elastic IP** is a **static public IP address** that you allocate manually and attach to your EC2 instance.

Unlike regular public IPs:

* It does NOT change when you stop/start the instance
    
* It belongs to your AWS account
    
* You can remap it to another instance
    

Elastic IP solves the “changing public IP” problem.

### Why “Elastic”?

Because you can:

* Detach it from one instance
    
* Attach it to another instance instantly
    

This is useful in:

* Disaster recovery
    
* Failover setups
    
* Production environments
    

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## Quick Comparison Table

| Feature | Private IP | Public IP | Elastic IP |
| --- | --- | --- | --- |
| Internet Accessible | ❌ No | ✅ Yes | ✅ Yes |
| Static | ✅ Yes | ❌ No | ✅ Yes |
| Used For | Internal communication | Basic internet access | Production-grade public access |
| Changes on Restart | ❌ No | ✅ Yes | ❌ No |
| Extra Cost | ❌ No | ❌ No | ⚠️ Yes (if unused) |

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## Real-World Use Case Example

Let’s say you're deploying a production application:

* Load Balancer → Public access
    
* EC2 App Servers → Private IP only
    
* RDS Database → Private IP only
    

In some cases:

* You attach an Elastic IP to a Bastion Host
    
* Or attach an Elastic IP to a production EC2 server
    

This setup improves both security and reliability.

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## Cost Considerations (Important)

Elastic IPs are free **only when attached to a running instance**.

AWS charges you if:

* You allocate an Elastic IP, but don’t use it
    
* You attach more than one Elastic IP per instance (in some cases)
    

Always release unused Elastic IPs to avoid charges.

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## Security Perspective

Best practice in AWS architecture:

* ❌ Never expose databases with a public IP
    
* ❌ Avoid unnecessary public IP assignments
    
* ✅ Use private subnets for backend services
    
* ✅ Use Elastic IP only when you truly need static public access
    

Security Groups and NACLs still control traffic regardless of IP type.

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## Final Thoughts

Understanding Private, Public, and Elastic IPs is foundational for AWS networking.

If you remember just one thing:

* Private IP → Internal communication
    
* Public IP → Temporary internet access
    
* Elastic IP → Permanent public identity
    

Once you master this, VPC architecture becomes much easier to design and troubleshoot.

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Happy Learning 🚀