🌐 Public vs Private IP & NAT

1. Public IP Address

• Definition: An IP address assigned by your Internet Service Provider (ISP) that is visible on the Internet.
• Scope: Globally unique, registered with organizations like InterNIC.
• Use case: Websites, servers, or any device that must be directly accessible online.
• Example ranges: Any IP not reserved for private use (e.g., 8.8.8.8 for Google DNS).
• Pros: Enables direct communication across the Internet.
• Cons: Less secure, consumes limited IPv4 address space.

2. Private IP Address

• Definition: IP addresses reserved for use within local networks (LAN/WLAN).
• Scope: Not routable on the Internet; only valid inside the private network.
• Common ranges (IPv4):
  • 10.0.0.0 – 10.255.255.255
  • 172.16.0.0 – 172.31.255.255
  • 192.168.0.0 – 192.168.255.255
• Use case: Home Wi‑Fi devices, office computers, printers.
• Pros: Conserves public IPs, more secure.
• Cons: Needs NAT to access the Internet.

3. NAT (Network Address Translation)

• Definition: A process where a router translates private IP addresses into a public IP address for Internet communication.
• How it works:
  • Your laptop (192.168.1.5) sends a request → router replaces it with the public IP (e.g., 203.0.113.25).
  • The router keeps a translation table to know which private device made the request.
• Types of NAT:
  • Static NAT: One private IP ↔ one public IP.
  • Dynamic NAT: Private IPs mapped to a pool of public IPs.
  • PAT (Port Address Translation): Many private IPs share one public IP using different port numbers (most common in home routers).
• Benefits:
  • Saves IPv4 addresses.
  • Adds a layer of security (internal devices aren’t directly exposed).
• Drawback: Can complicate peer‑to‑peer apps or hosting servers.

4. Putting It Together

• Private IPs are used inside your home/office LAN.
• Public IPs are used to communicate on the Internet.
• NAT bridges the two, allowing multiple private devices to share a single public IP.