When I set up Google's free DNS service for my home network, I wanted to be able to use any public DNS to look up my home machines. For example, if my coffee maker is called coffee.dammitly.net, I wanted to be able to type that address into the web browser on my phone.
My plan was to just use one of the private IP spaces like 192.168.0.0/16 or 10.0.0.0/8 to number my home network, and have Google DNS resolve my office light bulb to one of these addresses.
Unfortunately, Google's free DNS service won't let you enter private IP address A records into its database. I would need to use public IP addresses. The problem was, those are impossible to get now with ipv4 address exhaustion.
Sure, I could re-number my entire network using the new ipv6 standard and then set up SLAAC or DHCP, but that's a lot of work for my small network.
I decided to go searching to see if there were any public IP blocks I could poach for my home network. A little research turned up the 22.214.171.124/8 subnet, assigned to Amateur Radio's AMPRNet. This is a block of 16 million public addresses for Amateur Radio devices to use. More research showed that most of these weren't even assigned to routable devices.
In other words, a person could use this public IP block on any local network with little concern for preventing their users from accessing other important services. If you know your users will not be connecting to amateur radio devices (and most probably won't), you've got a free block of 16 million addresses.
The best part, Google's free DNS service allows you to create Address (A) Records with these addresses. I can now look up my thermostat's IP address from anywhere, and I didn't have to set up my own DNS server.