TLD stands for "Top-Level
Domain". in the taxonomy of the
Internet's DNS name space it is the
part of a domain name that is furthest
to the right of a character based,
human-readable address string.
Most people are familiar with TLDs
such as .COM; .NET; .ORG marketed
as a "Generic" TLD (gTLD)
as opposed to a "Country Code"
TLD (ccTLD) like .AU; (for Australia);
.UK (for Great Britain); and .TV (For
Several thousands of TLDs exist -
not just the gTLDs and ccTLDs - and
the Public-Root resolves all these
TLDs on a global scale planet.
An "SLD" is a "Second-Level
Domain", and is commonly recognized
by some sort of associative string
that indicates it's respective registrant
in the form of a company name, product,
Examples of SLDs that most people
are familiar with are "IBM",
"MapQuest", or "Yahoo"
and appear in context with their 'parent'
zone, the TLD it is anchored to: i.e.,
IBM.COM, MapQuest.COM, and Yahoo.COM.
Many countries organize their respective
domain space by creating a domain
space with generic SLDs under their
ccTLD. Great Britain and Australia
are examples of ccTLD operators that
specify such organizational standards
in the naming conventions for the
DNS zones they administer.
For example, YellowPages.COM.AU,
or IBM.CO.UK are examples of this
sort of organizational structure.
In such schematas, one cannot simply
register an SLD because the SLDs are
closed zones that are themselves registries
for 3LDs, or, "3rd-Level Domains".
3LDs do not necessarily have to adhere
to a conventional standard specified
by the governmental administrator/operator
of a ccTLD. They can occur under gTLDs
as well, and in fact many organizations
and even small companies incorporate
the use of 3LDs (and even 4LDs and
beyond) in their corporate or university
naming hierarchies. Examples of this
where 'barney' is the hostname of
the machine, ochem-lab is probably
the Organic Chemistry laboratory in
the math and physics building at the
University of California in Los Angeles.
The example above demonstrates the
hierarchy of the domain space, and
in this case is indicative of a structure
that utilizes a hostname, 4LD, 3LD,
and SLD, under the .EDU TLD. Although
college students have been accustomed
to such organizational conventions
in the DNS namespace for years, most
average consumers are usually only
likely to run across hostnames under
SLDs such as www.IBM.com, www.Google.com,
etc., the hostname in both instances
here being "www", and the
SLDs under the .COM TLD being, "IBM",
and "Google", respectively.