For me this happened today when learning a detail of the loopback address. Everyone with basic network knowledge knows that the IPv4 address 127.0.0.1 represents your local host and that you can reach your locally running network servers there. What was new to me is, that by definition, the whole network - 127.0.0.0/8 - is looped back to the local machine by definition.
The practical use of this is that software running locally can bind to any of these addresses and will only receive traffic for this exact address. So you can have a service running on 127.0.0.1 and another one at 127.0.0.81.
This alone is not super useful, but combined with adding these addresses to the hostname lookup and then using separate SSL certificates you can simulate different hosts on the local machine.
Happy networking!
(*) roughly 16 milliion