If you live in a typical household, you likely have a smartphone, a laptop, a smart TV, and maybe a gaming console all connected to the internet at the same time. All of these devices share a single public IP address provided by your ISP. So, when you load a video on your phone, how does the router know to send the video to your phone and not to your smart TV?
The magic behind this is a process called Network Address Translation (NAT).
Private vs. Public IP Addresses
Your router creates a private, local network inside your house. It assigns a private IP address (usually starting with 192.168... or 10.0...) to every device connected to the Wi-Fi. However, these private addresses are not routable on the public internet.
The Role of NAT
When you tap a video link on your phone, your phone sends a request to the router. The router looks at the request and does a few things:
- It records your phone's private IP address in a translation table.
- It replaces your phone's private IP with the router's public IP address.
- It assigns a unique "port number" to the request and sends it out to the internet.
To the video server, the request looks like it came directly from the router. The server sends the video data back to the router's public IP address, tagged with that specific port number.
Delivering the Package
When the video data arrives at your house, the router checks the port number on the incoming data, looks at its translation table, and says, "Ah, port 55432 matches the request made by the smartphone at private IP 192.168.1.5." It then forwards the data to your phone.
This entire process of translating addresses and checking ports happens in milliseconds for thousands of packets every second, ensuring everyone in the house gets the correct data without interference.