Last week, I did a Whiteboard Friday, The Microsite Mistake, in which I called out a practice I see as potentially detrimental to SEO - using a separate domain to accumulate links for your content site. I was not arguing against all microsite strategies or all domain separations or even all subdomains, just pointing out that this particular usage wasn't a very logical one. Then came the comments...

Don't get me wrong - I absolutely love the vibrancy and passion and level of engagement from all our readers and commenters, particularly those that disagree. Honestly, I believe that it is when we disagree and confront one another in positive, constructive ways that we learn the most so please, keep it up! In this instance, though, the comments weren't a particularly good place for me to address all the many thoughts that arose - that takes a post, and here it is.

Root domains vs. Subdomains vs. Subfolders

What is a Root Domain?

• Root Domains - the domain name you need to buy/register with a TLD extension
  • Examples of root domains:
    • *.seomoz.org
    • *.searchengineland.com
    • *.blogspot.com
    • *.about.com

What is a Subdomain?

• Subdomains - the "third level" domain name; these are free to create under any root domain you own/control
  • Examples of subdomains
    • www.seomoz.org
    • searchengineland.com (yes! even without a third-level name it falls under our definition of a subdomain in this application)
    • postsecret.blogspot.com
    • southernfood.about.com

What is a Subfolder?

• Subfolders - the folders behind a domain address
  • Examples of subfolders
    • www.seomoz.org/blog/
    • searchengineland.com/columns/
    • postsecret.blogspot.com/2009/
    • southernfood.about.com/library/

Search engines have metrics that they apply to pages, such as PageRank, and metrics they apply to subdomains and root domains (including things like TrustRank, various quality scores, domain level link metrics like Domain mozRank, etc.). Through years of experience, observation and testing, SEOs have observed some very steady patterns of behavior:

• Individual pages benefit from being on powerful subdomains & root domains. This is why if someone copies your personal blog post on the best way to microwave burritos into Wikipedia, that page will rank far better than yours, even with the exact same content (ignoring the duplicate content issues).
• Subdomains DO NOT always inherit all of the positive metrics and ranking ability of other subdomains on a given root domain.
• Some subdomains GET NO BENEFIT from the root domain they're on. These include sites like Wordpress.com, Blogspot.com, Typepad.com, and many others where anyone can create their own subdomain to begin publishing.
• Subfolders DO appear to receive all the benefits of the subdomain they're on and content/pages behave remarkably similarly no matter what subfolder under a given subdomain they're put in.
• Good internal and cross-linking CAN HELP to give share the positive metrics from one subdomain to another (but not always and not perfectly).

For these reasons, if you're seeking to maximize your ranking ability for a given piece of content, it's my personal belief that you should, most of the time, keep it on 1 subdomain under 1 root domain (but feel free to use subfolders as it makes sense). Starting a blog? I almost always recommend yoursite.com/blog over blog.yoursite.com. Want to launch a new section of content? Use yoursite.com/newstuff rather than newstuff.yoursite.com.

However, there are exceptions...

When to Use a Subdomain

Subdomains can sometimes make sense when:

• You already have two pages from your main domain ranking for a particular search query (and are trying to saturate the search results with your listings). This works because Google will show a maximum of two URLs on a given search results pages from a given subdomain, but may show more from a given root domain if there are multiple subdomains. You can see Aaron Wall doing a great job with these technique here.
• You have a particular keyword you want to rank for that you're using in the subdomain (or a combination keyword phrase that the subdomain + root domain tie together perfectly) and you're doing specific targeting with the tactic of letting the copy/paste of the URL serve as ideal anchor text. For example, if I owned watch-reviews.com and used subdomains for specific brands like rolex.watch-reviews.com knowing that many people would link using the subdomain URL and give my page that perfect anchor text.
• You already have a subdomain that's working well, ranking well and would be a pain to move. In the past, we've done some work to redirect subdomains back to subfolders on a root domain and seen considerable rises in traffic & rankings, but this is almost universally for root domains with large numbers of subdomains. If you just have 1-5 subdomains and they're performing well, it's not a huge concern (though it might warrant testing a redirect on one just to see).

There are other situations, some of them more technical in nature, where it can make sense, but the best practice is to use one subdomain on a root domain for all your content. In my experience, unless you've got some serious SEO savvy and know exactly what you're doing and why this should be the default.

When it Pays to Use Microsites

Like subdomains, microsites, too have their place:

• If you're launching a product/service/business that you want to potentially sell off or brand completely differently from your main site/business, microsites or even separate macro-sites make sense. It's easy to sell off a domain and the business beneath it, but much harder if it's in a subdomain or subfolder.
• If you're releasing a product/service/promotion that you don't want people to know is associated with your site/brand and are prepared for the fact that your existing root domain/subdomain metrics won't help that content rank in the engines.
• If you have an exact match domain name for a particular keyword you're targeting, microsites can be powerful. Google's preference for and ranking boost was given to exact-match domains is a very powerful tool to use for SEO.
• If you're pushing a content piece that has little to no association with your site and you don't want the potential branding confusion or commercial association to hinder link & user growth. Just be wary - if you do this a lot and it's clear as a method to attract links that you'll then 301 redirect back to your site, you can cross the "spam/manipulation" line with the engines and lose out on the value.

There are probably several other good uses for microsites and times when it pays to apply them, but they tend to fit with some of the above principles. The big trouble with microsites is that they inherit none of the trust, authority, ranking power, consideration, etc. that search engines give to established, well-linked properties. Mistaking a link between two domains as a signal that the engines can interpret to mean "oh, these two sites are owned by the same company, and since I trust that one, I'll trust this one" can make for very unhappy execs when it comes time to see the new project's metrics.

As always, I'm looking forward to your opinions and experiences on this topic.