When we are talking about computers, I see both hostname and host name being used. Which is more proper? Should I put the space in there?
Where two words can be shown either separately or joined, as in host name and hostname, the two may exist side by side until one eventually becomes more popular. We have already seen the emergence of login as a single word, and the same may well happen here. The two citations in the Oxford English Dictionary, both from 1997, show host name. There are five records in the Corpus of Contemporary American English for host name, but only one for hostname.
Which you use yourself depends on any preference your organization may have and on the expectations of your readers. If in doubt, it’s probably wiser to take the conservative approach for the moment, and write host name.
In most technical settings, hostname is used — I would stick to that if you are writing for a technologically savvy audience, as host name can mean "the name of the host" which is not necessarily always synonymous.
To clarify somewhat on another answer posted here; 'login' may now be recognized as one word, but that doesn't mean that it's not also two words. (login [n], log in [v]).
Similarly, 'hostname' is unquestionably a single word in the tech kingdom; just because a dictionary (does/did) not recognize that word does not mean it is wrong.
Epigram'd: You're unlikely to see
grep in a dictionary, but that is unquestionably the proper spelling of that tech word.
In the context of computer, hostname is the one I've always used and the one I've mostly read for the last years. Google agreed with more than 44 millions results for hostname and less than 10 millions for host name.
In your research on usage, be careful that results from the era before computer networking became common, the only valid use of "host name" was about social functions.
Hostname specifically refers to a particular computer or setting on a computer or setting in name server records. Host name is used when talking about the subject but not referring directly to a specific setting of it.
If I was writing a guide for someone to set up a name for their computer, I'd refer to it as a host name. The host name of a server you connect to (to view a web page) is a variable called hostname.
RFC814 (http://www.rfc-editor.org/rfc/rfc814.txt) uses "host name"
More recent RFC's use "hostname"