Hostnames are domains. A domain is a hierarchical, dot-separated list of subdomains. For example, the machine monet, in the Berkeley subdomain of the EDU subdomain of the Internet Domain Name System would be represented as monet.Berkeley.EDU

The hostname command shows or sets the system's host name.

hostname   returns the system's hostname :
localhost.localdomain is the string returned on a system that was newly created

hostname methane.Jupiter sets the hostname to methane.Jupiter

hostname -s (short host name) returns the first part of the hostname (to the first dot)
methane would be returned in this case

(Note - BSD does not have this option)
hostname -i returns the ip address for the host  was returned here

see ifconfig to configure a network interface