Private vs Public vs Elastic IP · 03

IPv4 vs IPv6

Feature	IPv4	IPv6
Format	`192.168.1.1` (4 numbers, dots)	`2001:0db8:85a3::8a2e:0370:7334`
Addresses	~3.7 billion	Virtually unlimited
Common Use	Most common online	IoT, future-proofing
AWS Support	✅ Full	✅ Full

Course Focus

This course focuses on IPv4 as it's still the most common format used online.

Public vs Private IP

┌─────────────────────────────────────────────────────────────┐
│                        INTERNET                              │
│                     (Public Network)                         │
└──────────────────────────┬──────────────────────────────────┘
                           │
              Public IP: 54.123.45.67
                           │
                    ┌──────┴──────┐
                    │   Internet  │
                    │   Gateway   │
                    └──────┬──────┘
                           │
┌──────────────────────────┴──────────────────────────────────┐
│                   PRIVATE NETWORK (VPC)                      │
│                                                              │
│   ┌─────────────┐    ┌─────────────┐    ┌─────────────┐     │
│   │ EC2 Instance│    │ EC2 Instance│    │ EC2 Instance│     │
│   │ 10.0.1.10   │    │ 10.0.1.11   │    │ 10.0.1.12   │     │
│   └─────────────┘    └─────────────┘    └─────────────┘     │
│                                                              │
│              Private IPs: 10.0.0.0/16 range                  │
└──────────────────────────────────────────────────────────────┘

Public IP Characteristics

Feature	Description
Uniqueness	Globally unique across entire internet
Accessibility	Machine accessible from anywhere on internet
Geolocation	Can be looked up to find approximate location
Assignment	Assigned by AWS, changes on stop/start

Private IP Characteristics

Feature	Description
Uniqueness	Unique only within private network
Accessibility	Only accessible within the VPC
Reuse	Same private IP can exist in different networks
Persistence	Does not change on stop/start

Private IP Ranges (RFC 1918)

10.0.0.0    - 10.255.255.255   (10.0.0.0/8)     - Large networks
172.16.0.0  - 172.31.255.255   (172.16.0.0/12)  - Medium networks
192.168.0.0 - 192.168.255.255  (192.168.0.0/16) - Small networks

Public IP vs private IP

Private IPs are only reachable inside the VPC. Traffic to and from the internet goes through the Internet Gateway.

EC2 Instance IP Behavior

Default Configuration

Every EC2 instance gets:

Private IP: For internal AWS network communication
Public IP: For WWW access (if enabled)

SSH Access

From Your Computer:
├── Can use Public IP ✓ (via Internet)
├── Cannot use Private IP ✗ (not on same network)
└── Exception: VPN connection to VPC

Stop/Start Behavior

Public IP Changes!

When you stop and start an instance, the Public IP may change. The Private IP stays the same.

Before Stop:
├── Public IP:  54.123.45.67
└── Private IP: 10.0.1.10

After Start:
├── Public IP:  3.250.12.89   ← CHANGED!
└── Private IP: 10.0.1.10     ← Same

Which IP changes on stop/start

The public IP may change after stop and start. The private IP stays, and an Elastic IP is static.

Elastic IP

Static Public IP

An Elastic IP is a static public IPv4 address that you own until you release it.

Characteristics

Feature	Details
Ownership	You own it until you delete/release it
Attachment	Can attach to one instance at a time
Portability	Can move between instances for failover
Limit	5 per region (can request increase)
Cost	$0.005/hour (~$3.60/month) whether used or not

Use Case: Failover

Normal Operation:
┌─────────────┐
│ Elastic IP  │──────▶ EC2 Instance A (Primary)
│ 54.1.2.3    │
└─────────────┘

After Failover:
┌─────────────┐
│ Elastic IP  │──────▶ EC2 Instance B (Backup)
│ 54.1.2.3    │        (Same IP, different instance)
└─────────────┘

Elastic IP Workflow

1. Allocate Elastic IP
   EC2 Console → Elastic IPs → Allocate Elastic IP address

2. Associate with Instance
   Actions → Associate Elastic IP address → Select Instance

3. Verify
   Instance now shows Elastic IP as Public IPv4

4. Stop/Start Test
   Public IP remains the same! ✓

5. Cleanup (Important!)
   Disassociate → Release Elastic IP address

Best Practices

Avoid Elastic IPs When Possible

Elastic IPs are often considered poor architectural decisions.

Better Alternatives

Instead of Elastic IP	Use
Static IP for web server	DNS name (Route 53)
Direct instance access	Load Balancer
Failover	Auto Scaling Group

Why Avoid Elastic IPs?

Limited: Only 5 per region
Cost: Charged even when not attached
Not scalable: One IP = one instance
Better patterns exist: DNS, Load Balancers

Pricing (Since Feb 2024)

IPv4 Charges

All Public IPv4 addresses now cost $0.005/hour (~$3.60/month)

Scenario	Cost
EC2 with Public IP (running)	$0.005/hour
Elastic IP (attached, running)	$0.005/hour
Elastic IP (not attached)	$0.005/hour
EC2 Free Tier	750 hours/month free (first 12 months)

Questions & Answers

Q1: What happens to the Public IP when you stop and start an EC2 instance?

Answer:

The Public IP may change when you stop and start an instance. The Private IP stays the same. To keep a static public IP, use an Elastic IP.

Q2: Can you SSH into an EC2 instance using its Private IP from your home computer?

Answer:

No, unless you have a VPN connection to the VPC. Private IPs are only accessible within the private network. You must use the Public IP to SSH from the internet.

Q3: What is an Elastic IP and when would you use it?

Answer:

An Elastic IP is a static public IPv4 that you own. Use cases:

Need a fixed IP that doesn't change on stop/start
Quick failover by moving IP between instances

However, better alternatives exist (DNS, Load Balancers).

Q4: How many Elastic IPs can you have per region?

Answer:

5 Elastic IPs per region by default. You can request an increase from AWS, but it's rare to need more. Consider using DNS or Load Balancers instead.

Q5: Are Elastic IPs free if attached to a running instance?

Answer:

No. Since February 2024, all Public IPv4 addresses (including Elastic IPs) cost $0.005/hour whether attached or not. Always release unused Elastic IPs.

Q6: What is the difference between IPv4 and IPv6 in AWS?

Answer:

IPv4 uses 4 numbers separated by dots (e.g., 192.168.1.1) with ~3.7 billion addresses. IPv6 uses a longer hex format and is virtually unlimited. AWS supports both, but IPv4 is still the most common. IPv6 is mainly used for IoT.

Q7: How do private machines connect to the internet?

Answer:

Machines on a private network connect to the internet through a NAT device and an Internet Gateway that acts as a proxy. The Internet Gateway provides a public-facing IP for outbound traffic.

Q8: Can two different companies have the same private IP addresses?

Answer:

Yes. Private IPs only need to be unique within their own private network. Two separate VPCs or companies can use identical private IP ranges (e.g., both using 10.0.1.10) without conflict.

Q9: What are the recommended alternatives to Elastic IPs?

Answer:

Use a random public IP with a DNS name (Route 53) — more scalable
Use a Load Balancer and avoid public IPs on instances entirely — best pattern for AWS

Both approaches are more scalable and cost-effective than Elastic IPs.

Q10: What happens to an Elastic IP when you stop an instance it's attached to?

Answer:

The Elastic IP remains attached to the stopped instance. The public IPv4 does not change. When you start the instance again, the same Elastic IP is still associated. You must disassociate and release the Elastic IP manually to stop being charged.

Check yourself

Quiz

Flashcards

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