<jou128@yahoo.com> wrote:

: why some site can be
: accessed via http://some-site.com but not www.some-site.com?

My understanding is that once you own a domain MAINDOMAIN.com, it is up
to you to define any subdomains like SUBDOMAIN.MAINDOMAIN.com, if you
wish to. There is no requirement to do so.

So, let's say Harvard has registered harvard.edu . Now it is up to
them, if they want, to define subdomains like admissions.harvard.edu,
music.harvard.edu, history.harvard.edu, etc., including
www.harvard.edu.

By tradition, most domain owners have defined a "www" subdomain, but
there is no requirement and some don't do it.

Also by tradition, domain.com and www.domain.com usually lead to the
same page, but they need not.