The Art of SEO, 2nd Edition

Root Domains, Subdomains, and Microsites

Among the common questions that arise when 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 metrics:

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://www.yourdomain.com/page.html.

Subfolders

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://www.yourdomain.com/blog/post17.html, /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 http://blog.yourdomain.com/page.html, three kinds of domain levels are present. ...