The default hostname is: raspberrypi and has nothing to do with the IP address.
You can change it or leave it as it is.
You see the hostname at the command prompt when you start a terminal session.
Looks like this:
I have 2 RPi's in use, so I changed the hostname on one of them so when I am connected remotely, I don't forget which one I am logged into.
Unless specified otherwise my response is based on the latest and fully updated Raspbian Buster w/ Desktop OS.