NAT Explained: How Your Router Shares One IP Address
Network Address Translation is the clever technology that allows dozens of devices in your home to share a single internet connection.
The Clever Workaround
Your Internet Service Provider only gives your home a single public IP address. Yet, you likely have a smartphone, a laptop, a smart TV, and a gaming console all browsing the internet at the exact same time. How do all these devices share one public address without their data getting mixed up?
The answer is Network Address Translation (NAT), a core function built into your home router. Your router assigns private, fake IP addresses (usually starting with 192.168) to every device in your house. These private addresses only exist inside your walls.
Sorting the Mail
When your phone requests a webpage, it sends the request to the router. The router modifies the packet, swapping your phone's private address for the home's single public address, and makes a note of this swap in its NAT table. When the webpage data comes back, the router checks its table, finds out the data belongs to your phone, changes the address back, and delivers it.
Internal Network Router Public Internet Phone (192.168.1.5) ---> NAT Engine ---> Server (Sees Public IP)
Why This Affects Your Speed Test
NAT is a highly intensive computational process. For every single packet of data that enters or leaves your home, the router's processor must rewrite the headers and update its tracking table. During a high-speed Gigabit download test, your router is processing hundreds of thousands of packets per second.
If you have an older, cheap router attempting to route a Gigabit fiber connection, the router's CPU will max out at 100%. This hardware bottleneck will limit your speed test results and artificially increase your latency. A powerful router is required to translate addresses at multi-gigabit speeds.