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NAT is a method that translates public and private IP.
It's a built-in feature for most routers.
But why do we need NAT?
Or, why do we need both public and private IPs?
Why NAT?
Every device needs an IP address to be accessible on the Internet.
Previously, we've been using this IP format called IPv4.
And there are not enough of them.
Public IP has to be unique, and, in total there are only ~4 billion public IPv4 addresses. There are more devices than that.
So, we can't assign a public IP for every device.
This is why we need to use private IP addresses.
Public & Private IPs
Imagine this is your home network (a router with several devices)
We only assign one public IP to the router.
For all devices in that local network, we give them a private IP address instead.
Private IP addresses is reusable, while public IP isn't. So your neighbour's phone can use the same private IP.
But, your devices still need a public IP to be on the Internet.
This is where NAT comes in.
How does NAT work
When your device needs to send a request out...
We translate that private IP into a public one using NAT.
Then, when an external service needs to send a response to your device...
They send it to that port of the public IP address of your router, and the router will translate it to your private IP using a NAT table.
Port Forwarding
There's a problem: NAT table changes frequently.
When a connection to your device is unused, NAT deletes that row to free up the public port for other devices.
What this means is an external device simply cannot initiate a connection to your local device, because it doesn't know what your device's IP is at any given moment.
Unless you use something like port forwarding.
Basically, port forwarding creates a fixed row on the NAT table. Like this:
Now, the external device has a reliable way of reaching your local device.
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