There is no automatic propagation of host names, as has been repeated several times.
During installation when you specify your hostname as "bilbo" no other system is made aware of this name, systems on your internal network must list this hostname in their /etc/hosts file or it must be added to a local resolver.. for example your router might intercept requests for "shire.middleearth" and if "bilbo" is known it would return the IP.
Before the existence of DNS servers all known host/domain names were distributed in a large
/etc/hosts file, "hosts" were publicly addressable on the Internet as part of a "domain", for example a university or college.
Also, I'd recommend not using underslashes as part of a host or domain name as their violation of the RFC specifications (..which you may wish to read), basically just avoid choosing a domain name that is on the public Internet, as you'll be unable to reach them.
A small helpful man page exists that attempts to explain the concept of "hostnames" which themselves are just special forms of "domain names", see
hostname(7).