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

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.