7. Private and public address... Nat gateway-device

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🌐 **Private IP, Public IP & NAT Gateway **

• The college has many private IPs inside the campus LAN:

  10.x.x.x 172.16.x.x 192.168.x.x

  These are private IP ranges (not accessible from the Internet).

• All devices — students’ laptops, lab PCs, Wi-Fi users — connect to the Internet through a central router or firewall.

• That router performs NAT (Network Address Translation).
  It replaces each private IP (like 192.168.1.10) with the college’s single public IP when traffic goes to the Internet.

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1️⃣ Public IP

✔ Definition:
A Public IP is an address that’s accessible over the Internet.
It’s assigned to resources that need to communicate outside the private network (e.g., web servers).

✔ Key Points:

• Unique across the entire internet 🌍
• Provided by ISP or cloud provider (like AWS)
• Can be static (Elastic IP) or dynamic
• Used for direct external communication

🧠 Example:
52.14.22.101 → Public IP of an EC2 instance accessible from your browser.

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2️⃣ Private IP

✔ Definition:
A Private IP is used for internal communication within a private network (VPC, LAN).
It cannot be accessed directly from the Internet.

✔ Key Points:

• Defined by RFC 1918 ranges:
  • 10.0.0.0 – 10.255.255.255
  • 172.16.0.0 – 172.31.255.255
  • 192.168.0.0 – 192.168.255.255
• Used for communication between internal servers (e.g., app → database)
• Cheaper and more secure than using public IPs

🧠 Example:
10.0.2.15 → Private IP used inside a VPC subnet.

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3️⃣ NAT Gateway (Network Address Translation)

✔ Definition:
A NAT Gateway allows instances in private subnets to access the Internet (for updates, APIs, etc.)
➡️ while preventing inbound traffic from the Internet.

✔ How It Works:

1. Private instance → sends request to Internet
2. NAT Gateway → replaces private IP with its public IP
3. Response → comes back to NAT Gateway → forwarded to private instance

✔ Key Points:

• Deployed in a public subnet
• Assigned a public Elastic IP
• One-way communication: Outbound only
• Used for security + outbound internet access

🧠 Example Use Case:
Private EC2 → needs to yum update → request goes via NAT Gateway → to Internet.

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💡 Quick Comparison

Feature	Public IP	Private IP
Internet Access	✅ Direct	❌ No
Used In	Public Subnet	Private Subnet
Security	Less Secure	More Secure
Visibility	Global	Local

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