Inet Domain Name, Help?

[Ron Reaugh]

What exactly is a registerable Internet domain name and how does that relate
to subdomains?  For instance if one owns or is renting  xyzabc.com then
what exactly is jkl.xyzabc.com?  I understand that the jkl is known as a
subdomain.  But is the complete entity jkl.xyzabc.com something that is
independently registerable from xzyabc.com?  OR if one owns xyzabc.com then
does one own automatically all subdomains off xyzabc.com in Internet domain
name space?  I guess the question boils down to whether '.' is a special and
non-registerable character below the TLD level and therefore the
registerable domain name is that entity in the right most portion of the
name including one and only one
'.' which preceeds a valid TLD?