Root Domains, Subdomains, and Microsites

Among the common questions in structuring a website (or restructuring one) are whether to
host content on a new domain, when to use subfolders, and when to employ microsites.
As search engines scour the Web, they identify four kinds of web structures on which to place

Individual pages/URLs
These are the most basic elements of the Web—filenames, much like those that have been
found on computers for decades, which indicate unique documents. Search engines assign
query-independent scores—most famously, Google’s PageRank—to URLs and judge them
in their ranking algorithms. A typical URL might look something like http://

The folder structures that websites use can also inherit or be assigned metrics by search
engines (though there’s very little information to suggest that they are used one way or
another). Luckily, they are an easy structure to understand. In the URL http://, “/blog/” is the subfolder and “post17.html” is the
name of the file in that subfolder. Engines may identify common features of documents
in a given subfolder and assign metrics to these such as how frequently the content
changes, how important these documents are in general, or how unique the content is
that exists in these subfolders.

Subdomains/fully qualified domains (FQDs)/third-level domains
In the URL, three kinds of domain levels are present.
The top-level domain (also called the TLD or domain extension) is “.com”, the second-level
domain is “yourdomain”, and the third-level domain is “blog”. The third-level domain is
sometimes referred to as a subdomain. Common web nomenclature does not typically apply
the word subdomain when referring to www, although technically, this too is a subdomain.
A fully qualified domain is the combination of the elements required to identify the
location of the server where the content can be found (in this example,
These structures can receive individual assignments of importance, trustworthiness, and
value from the engines, independent of their second-level domains, particularly on hosted
publishing platforms such as WordPress, Blogspot, Wetpaint, and so on.

Complete root domains/host domain/pay-level domains (PLDs)/second-level domains
The domain name you need to register and pay for, and the one you point DNS settings
toward, is the second-level domain (though it is commonly improperly called the “toplevel”
domain). In the URL, “” is the
second-level domain. Other naming conventions may refer to this as the “root” or “paylevel”