Why was I told that xxx in the web address xxx.yyy.com is not a subdomain?

Question

edit: more to the address than I had given in the example. It has a subfolder?? https://some_external_website.com/bh/public

Is it correct to say that xxx is a subdomain of yyy.com written as xxx.yyy.com

email I sent to the BIG IT dept: names changed to protect the innocent
Additional Info:Please create an entry that will map the subdomain xxx.yyy.com to https://some_external_website.com/bh/public this is an externally hosted web application. Please call me if you have any questions.

--end of message--

About an hour later I get a call because they don't know what I want, I was told that xxx is not a subdomain. The correct definition subdomain it would have to be xxx.www.yyy.com.

Answer 1

The first component of a "domain name" is always the hostname. We can view a domain name as consisting of a hostname followed by one or more domain components. Each domain component is a subdomain of the component to it's immediate right. In xxx.yyy.zzz, xxx is the hostname (typically of a single machine, unless some kind of load balancing is going on), and yyy is a sub-domain of the zzz top-level domain. Colloquially we usually refer to zzz as the top-level domian, yyy as "the domain", and all other names to the left (excluding leftmost which is the hostname) as sub-domains. I'll add the disclaimer that I am by no means a DNS expert but to the best of my knowledge this would be why they aren't understanding your question. The hostname is not a "domain" per-se, i.e. it defines a single machine rather than a group (domain) of machines.