The Internet is built on what is called version 4 of Internet Protocol, which when they designed it allotted 4Bn total IP addresses. Each device on the Internet would be able to have one IP and 4Bn would be more than enough for the whole of Planet Earth.
Fast forward a couple of decades and everyone needs 10 IPs each. You have your phone, your laptop, your work computer, your TV, your door lock, your door bell camera, your thermostat, etc. And every web site in the world traditionally needed its own IP. And so the world pretty much ran out of those 4Bn addresses.
The "Powers That Be" developed IPv6 which solves this, but not everything works properly yet when connecting with IPv6, so if you want to make sure your software/hardware is guaranteed to connect to everything then you need one of those precious IPv4 addresses.
Now, in the early days of the Internet there were so many addresses that they were handed out like Halloween candy. And many people had so many they didn't even use a fraction of them. So now you can make good money selling your old addresses as they are prime real estate.
And the people who designed IPv6 designed a beautiful, perfect protocoll that was incompatible with the old IPv4 protocol. Which meant that a graceful, gradual shift to IPv6 was impossible. Cheap IoT devices are still being manufactured that support only the old protocol.
> Fast forward a couple of decades and everyone needs 10 IPs each. You have your phone, your laptop, your work computer, your TV, your door lock, your door bell camera, your thermostat, etc.
Your phone perhaps, but the rest of these devices never need a public IP address.
We should abandon the very idea of "public" and "private" addresses. An IP address is just an IP address, a globally-unique number that identifies a networked device. If you want a device to be inaccessible by other devices, throw in a firewall. NAT is just a firewall with packet-modifying capabilities anyways.
Your phone never needs (nor gets) a public IP either.
Pretty much every cell network gives the phone an IP on the subnet, and then uses NAT, or CG-NAT[1] to share the same public IPs for multiple mobile devices.
Fast forward a couple of decades and everyone needs 10 IPs each. You have your phone, your laptop, your work computer, your TV, your door lock, your door bell camera, your thermostat, etc. And every web site in the world traditionally needed its own IP. And so the world pretty much ran out of those 4Bn addresses.
The "Powers That Be" developed IPv6 which solves this, but not everything works properly yet when connecting with IPv6, so if you want to make sure your software/hardware is guaranteed to connect to everything then you need one of those precious IPv4 addresses.
Now, in the early days of the Internet there were so many addresses that they were handed out like Halloween candy. And many people had so many they didn't even use a fraction of them. So now you can make good money selling your old addresses as they are prime real estate.