How NAT Saved the Internet

Networking Part 1 DevOps Docs

Networking · IPv4 · NAT

Why IPv4 Ran Out of Addresses — and How NAT Saved the Internet

From 4.3 billion addresses to billions of devices: a practical walkthrough of IPv4 exhaustion, public vs private IPs, NAT types (Static, Dynamic, PAT), port forwarding, drawbacks, and IPv6.

On this page

• The Basics: What Is IPv4?
• Public vs Private IP Addresses
• The IPv4 Crunch
• Enter NAT
• Types of NAT
  • 1. Static NAT
  • 2. Dynamic NAT
  • 3. PAT (Port Address Translation)
  • Bonus: Port Forwarding
• Quick Comparison
• ASCII Diagram: PAT
• Drawbacks of NAT
• What About IPv6?
• Conclusion

When the Internet was designed in the 1970s, no one anticipated today’s billions of smartphones, smart TVs, and IoT devices. By the late 1990s, it was clear: IPv4 was running out of space. This is the story of how we hit the limits — and how Network Address Translation (NAT) kept things running.

The Basics: What Is IPv4?

• IPv4 stands for Internet Protocol version 4.
• It uses 32-bit addresses, written as dotted quads (e.g., 192.168.0.1).
• 32 bits yields ~4.3 billion unique addresses.

Why it wasn’t enough:

• Every device needs an address.
• Large early allocations to orgs/ISPs/universities.
• Many allocations were underused or wasted.

Public vs Private IP Addresses

1) Public IP addresses

• Globally unique and Internet‑routable.
• Assigned by regional registries (ARIN, RIPE, APNIC, etc.).
• Example: 8.8.8.8 (Google DNS).

2) Private IP addresses

• Reserved ranges, reusable inside local networks.
• Not routable on the public Internet.
• Ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16.

👉

Question: How do devices on private addresses talk to the wider Internet?

The IPv4 Crunch: Why 4.3 Billion Wasn’t Enough

• Inefficient allocation: Class A (/8) blocks gave 16M addresses each, often vastly over‑provisioned.
• Device explosion: Laptops, phones, tablets, IoT.
• Always‑on era: Devices stay online, consuming addresses continuously.
• Global scale: Billions of people needing connectivity.

Enter NAT: The Lifesaver

Network Address Translation (NAT) lets many devices share a single public IP by rewriting packet headers.

1. Inside your network, devices use private IPs (e.g., 192.168.1.10).
2. The router keeps a translation table.
3. Outbound traffic has its source rewritten to the public IP.
4. Replies consult the table to reach the correct internal device.

Invisible to users — essential to the modern Internet.

Types of NAT (with Examples)

1. Static NAT — One‑to‑One

Scenario: Host a web server at 10.0.0.10 using public IP 203.0.113.10.

[Internet] ──(203.0.113.10)── [NAT Router] ──(10.0.0.10)── [Web Server]

• Map 10.0.0.10 ↔ 203.0.113.10.
• Requests to public IP forward to the server; replies are rewritten.

10.0.0.10:80  <->  203.0.113.10:80

Use case: Hosting public services. Limit: Consumes one public IP per host.

2. Dynamic NAT — Many‑to‑Many (Pool)

Scenario: Office 10.0.1.0/24 with pool 203.0.113.20–22.

• Users are temporarily mapped to free public IPs from the pool.
• If the pool is exhausted, new connections wait.

10.0.1.11  <->  203.0.113.20
10.0.1.12  <->  203.0.113.21
10.0.1.13  <->  203.0.113.22

Use case: Older enterprise networks. Limit: Pool exhaustion.

3. PAT (Port Address Translation) — Many‑to‑One

Scenario: Home subnet 192.168.1.0/24 with public IP 198.51.100.23.

[Phone 192.168.1.11]     \
[Laptop 192.168.1.10] ---- [NAT Router] ---- (198.51.100.23) ---- [Internet]
[TV    192.168.1.12]     /

• 192.168.1.10:52344 → 198.51.100.23:40001
• 192.168.1.11:50123 → 198.51.100.23:40002
• Ports keep flows distinct in a single public IP.

192.168.1.10:52344  <->  198.51.100.23:40001
192.168.1.11:50123  <->  198.51.100.23:40002

Use case: Homes/SMBs. Limit: Inbound breaks unless port forwarding is configured.

Bonus: Port Forwarding with PAT

Expose an internal service selectively.

Example: SSH to 192.168.1.50:22 via public IP 198.51.100.23:

• Rule: 198.51.100.23:2222 → 192.168.1.50:22

ssh user@198.51.100.23 -p 2222

Quick Comparison

Type	Mapping	Use Case	Pros	Cons
Static NAT	One‑to‑one	Hosting servers with fixed IPs	Stable, predictable	Consumes a public IP per host
Dynamic NAT	Many‑to‑many	Older enterprise setups	Conceptually simple	Pool exhaustion possible
PAT	Many‑to‑one	Home & SMB networks	Conserves public IPs, scalable	Blocks inbound by default

ASCII Diagram: How PAT Works (Home Router)

Device A: 192.168.1.10:52344  --->
                                       NAT Router (Public IP 198.51.100.23)
Device B: 192.168.1.11:50123  --->
                                       NAT Table:
                                       192.168.1.10:52344 -> 198.51.100.23:40001
                                       192.168.1.11:50123 -> 198.51.100.23:40002

From Internet’s view:
198.51.100.23:40001 -> goes back to Device A
198.51.100.23:40002 -> goes back to Device B

The Drawbacks of NAT

• Breaks end‑to‑end connectivity: Inbound access requires port forwarding or application relays.
• Complicates protocols: VoIP and P2P often need helpers (ALGs, STUN/TURN/ICE).
• Adds overhead: Routers maintain translation tables and rewrite packets.

Still, NAT was far easier than re‑architecting the Internet overnight.

What About IPv6?

• IPv6 uses 128‑bit addresses — about 3.4 × 10^38 possibilities.
• Enough addresses for, figuratively, every grain of sand to have one.
• Adoption continues to grow, but IPv4 + NAT remain critical today.

Conclusion

IPv4 wasn’t built for today’s scale. NAT stepped in as a practical workaround so billions of devices could share far fewer public IPs. While IPv6 is the future, NAT is the technology that kept the Internet running during IPv4’s growing pains — and still powers most home/office networks you use every day.

Back to top ↑