As a professional Search Engine Optimizer, I am asked a lot of questions by people who don't know the first thing about computers let alone search engines. I am also asked questions of those lucky few who are new to the field, and among these so-called "newbies" a recurring question often pops up - Which of the following ways is best to structure site URLs?

• www.site.com/subfolder OR
• subdomain.site.com

In this article I will attempt to answer the question, “which URL structure is best in terms of SEO, subfolders or subdomains?"

Short Answer: If the breadth of an issue or keyphrase will be covered in such detail that a new website should be constructed to best accommodate the content, or more simply, if the site’s graphical navigation will need to be altered, use subdomains. In all other cases, use subfolders.

Long Answer: To answer this question in detail, we’ll need to go over a few key facts regarding subdomains, subfolders and site themes.

Subdomains:

• Represented in the URL by the characters after "http://" and the ones before ".site.com"
• Are viewed by Search Engines as totally unique sites independent of the parent domain.
• Site Authority (in other words, the ability for the site and its subsequent pages to rank in the search engines) is not passed down from the parent domain.
• Can be listed on the first page of the SERPs along with main domain pages. Feasibly, all 10 of the results on the first page of a search engine could be subdomains of a single parent domain.

Subfolders:

• Represented in the URL by the characters after ".site.com/" and before "filename.html".
• Are viewed by Search Engines as a subdirectory within a site.
• Site Authority is passed down from the main domain.
• At most, one subfolder can appear on the first SERP from the same domain or subdomain.

Site Theme:

• As a general rule, the more focused the theme of a site the easier it is for that site to rank for keywords directly related to its theme.
• Is defined using all of the site’s pages, not just the homepage or top subpages.

So here’s the long answer:

Search engine optimization is all about driving the most traffic. Where the goal is to build an authority site with lots of great content that will be found by search engine driven visitors using laser-target search terms, as well as unique, "long-tail" search terms that most would never think to optimize for, but appear on pages in the subfolders of your site and drive traffic just the same.

Using subdomains may appear sexy because they can lead to a potentially larger number of back-links (though not if each subdomain is hosted on the same IP), or they can sway the SERPs in your favor by crowding out the top positions with your pages. But without proper promotion, a subdomain only stops potential laser-targeted as well as "long-tail" search visitors from finding your site.

My advice is only to use subdomains when the amount of content on a given topic is so burgeoning that it requires a separate navigation and design (i.e. its own website), or is of great enough quality that it will acquire links and stand on its own.

For example:

• If you have a 15 page site about your lawn-care service, and 10 of those pages revolve around a special fertilizer you’ve developed, you may consider creating "specialfertilizer.site.com" so that you can market your fertilizer separately.
• Take my company’s domain "objectwareinc.com". Recently, we built a subdomain "palm.objectwareinc.com", because we had so much information regarding our wireless application development solutions that it only made sense to create a separate site for it.

In this example, the focus is narrow but there is still an extensive amount of content. So we’re sending visitors to the subdomain using targeted keywords while visitors are also finding us using "long-tail" searches and landing on the pages of palm.objectwareinc.com subfolders.

The overall lesson here is to create subdomains ONLY when you have the quality and/or quantity of content to support them. Remember that subdomains are independent sites in the eyes of the search engines, meaning that in order for them to rank, they will require promotion and link building efforts.