Without NAT, the Internet Would Have Collapsed in the 2000s

Your home has smartphones, PCs, tablets, smart speakers, game consoles, TVs, and many other devices connected to the Internet. Yet your ISP typically assigns you just one global IP address.

The magic that lets dozens of devices use the Internet with a single IP address - that's NAT (Network Address Translation). The IP address you see on IP Check-san is your router's global IP, and the reason it shows the same IP regardless of which home device you access from is thanks to NAT.

How NAT Works - Your Router's "Translation" Job

Devices in your home are assigned private IP addresses (192.168.x.x, 10.x.x.x, 172.16-31.x.x). These addresses cannot be used on the Internet. Your router manages a NAT table and "translates" between private and global IPs.

  1. Your PC (192.168.1.10) sends a request to a website
  2. The router converts the source address from 192.168.1.10 to the global IP (e.g., 203.0.113.5) and records the source port
  3. The website's response arrives at 203.0.113.5
  4. The router checks the NAT table and forwards the response to 192.168.1.10

This translation happens thousands of times per second, but users never notice it.

The Background of NAT's "Invention"

NAT was proposed in RFC 1631 in 1994. At the time, IPv4 address exhaustion was becoming a real concern, and NAT was designed as a "temporary stopgap until the next generation of IP becomes widespread." The first IPv6 specification, RFC 1883, was not published until December 1995, more than a year after NAT was proposed. Engineers needed to buy time before the replacement protocol had even been finalized.

However, NAT was so effective that it weakened the motivation to migrate to IPv6. The situation where "NAT makes IPv4 sufficient" has persisted for over 30 years. An ironic example of a temporary fix becoming permanent Internet infrastructure.

NAT's Side Effects - Security and Limitations

Unintended Firewall Effect

Devices behind NAT cannot be directly accessed from outside. Incoming communications without a corresponding NAT table entry are dropped by the router. This is a side effect rather than a design goal, but it effectively protects home devices from unauthorized external access.

P2P Communication Difficulties

Direct communication between two devices behind NAT is challenging. P2P communications like online gaming, video calls, and file sharing require NAT traversal using STUN/TURN servers. This is why WebRTC uses STUN servers.

CGNAT - NAT Within NAT

With CGNAT (Carrier-Grade NAT) applied at the ISP level, hundreds to thousands of customers share a single global IP. A single IP address offers only 65,535 usable port numbers, so the sharing works only by capping how many ports each customer may use at once. This causes IP reputation problems (one bad user gets everyone blocked) and reduced GeoIP accuracy.

Summary

NAT was born as an "emergency measure" for the IPv4 address exhaustion crisis and has supported the Internet for over 30 years. Behind the single IP address displayed on IP Check-san lie countless home devices managed by NAT.

Related Glossary Terms

NAT Technology that translates between private and global IPs. A stopgap for IPv4 address exhaustion. IP Address Refers to both the global and private IPs that NAT translates between. IPv6 The next-generation protocol that makes NAT unnecessary. Can assign global addresses to every device. Firewall NAT has the side effect of blocking unauthorized external access. ISP Operators that run CGNAT, sharing a single global IP among multiple customers.

Frequently Asked Questions

What is NAT?

NAT (Network Address Translation) allows multiple devices on a private network to share a single public IP address. Your router translates between private addresses (like 192.168.x.x) and the public IP assigned by your ISP, keeping track of which internal device each connection belongs to.

Does NAT help with security?

NAT provides an incidental security benefit by hiding internal network addresses and blocking unsolicited inbound connections. However, it was not designed as a security measure and should not replace a proper firewall. It does not inspect traffic content or prevent outbound threats.

What is CGNAT?

CGNAT (Carrier-Grade NAT) is NAT applied at the ISP level, where multiple customers share a single public IP address. This extends IPv4's life further but causes problems with online gaming, port forwarding, IP-based geolocation, and services that limit connections per IP address.