Just what is a hostname (sitename)?

A hostname (or sitename) is the unique name by which a network attached device (like a server or computer) is known on a network. The hostname is used to identify a particular host in various forms of electronic communication such as e-mail or Usenet. Hostnames are used by various naming systems, NIS, DNS, SMB, etc., and so the meaning of the word hostname will vary depending on naming system in question, which in turn varies by type of network. But generally a hostname is simply the name of your computer in a network, which means your computer has to be on a network, otherwise, you can't call it a hostname, bucause you no other hosts(computers) can see you, since the internet is a huge public network, so when you connect to the internet, you computer becomes a hostname.