What is a sub-domain -- host name?

[Kevin Darcy]

ojunk at optusnet.com.au wrote:

> Hi All
>
> Can anyone explain what the difference between a sub-domain and host
> name is?
> EG: is www.domain.com a subdomain of domain.com or is to refered to as
> a host name of domain.com?
>

"Subdomain" is just a relative form of the term "domain", and technically
every name in DNS is a "domain". Since the namespace is arranged
hierarchically, some names are at a different "height" in the
hierarchy than others, "subdomain" denotes that a particular name is
"lower" than some other name, e.g. "foo.example.com" is a subdomain of
"example.com".

In less strictly technical terms, people often reserve the terms "domain"
and "subdomain" for "branch" points in the namespace hierarchy, i.e.
names which have other names underneath them, and decline to use the
terms for "leaf" nodes.

As for "hostname", this term has a variety of meanings, but at the very
least it refers to the name by which a node on the network knows
*itself*, or by which it is known within a (perhaps limited) network
scope or context. Since a given node can exist on multiple networks, and
participate in multiple naming systems, it can have multiple hostnames.

Oftentimes, the hostname of general-purpose computer will consist of a
single label, e.g. "foo", and then this label will form the first part of
the FQDN (Fully-Qualified Domain Name) in DNS, e.g. "foo.example.com". In
this sense, there is an intersection between the terms "hostname" and
"subdomain" (or "domain")...

> To be a subdomain is it neccessary to be delegated and have a seperate
> zone file and SOA record to the parent domain?

No, it's not necessary. That constitutes a separate *zone*, but not every
subdomain needs to have a zone cut. To take things to ridiculous
extremes, the whole Internet namespace, for instance, could theoretically
be contained in a single zone.


- Kevin