Re: Underscore? What is a hostname?
Back when I was a lad the hostname was the leftmost part of the fully qualified domian name.
Red herring. That is not the usage of "hostname" in any of the references from here.
And there's nothing "modern" about it: the 'modern' bit is the whole notion of a FQDN running from local on the left to TLD on the right. Until sometime in the '90s, that would only ever have been considered one among many formats.