Question: Are sub-domains on a website considered to be part of that domain? Is there any advantage to using sub-domains over sub-directories, or sub-directories over sub-domains? Will it hurt my website to move one from the other?
The question of subdomains vs. subdirectories is a great example of SEO voodoo. According to Matt Cutts, using a subdomain is, to Google, materially identical to using a subdirectory. There’s no advantage to using one or the other except for purposes of ease-of-use or simple preference.
Yet many marketers will swear up and down that subdirectories are better.
In fact, the thought leaders at Moz go so far as to imply that Matt Cutts is lying by omission and that Google really does rank sub-domains differently from the domain.
“I think the important word you used in describing Matt’s video is “implied.” He’s very careful not to speak in specifics, and often, I think the truth is buried in that non-specific language, rather than in the broader implied phrasing.”
Because why not trick the people you’re trying to educate? Just for kicks.
The example Moz uses is moving a guide from http://guides.moz.com to http://moz.com/beginners-guide-to-SEO.
But that’s not comparable at all. A comparable move would be from beginners-guide-to-SEO.moz.com. They moved their guide from one non-optimized URL to an optimized URL; of course this would make a dramatic difference. It has nothing to do with domain structure.
It’s often repeated that subdomains are looked at as entirely different sites from the main domain. We know, from Google, that this is just not true — and there’s truly no reason for Google to lie about this. And many marketers state that they’ve moved content from a subdomain to a subdirectory and seen an incredible increase in traffic. But it’s very likely that this is due to some other change that they’ve made — or simply due to the fact that the content has then been up for longer.
Answer: Google has stated subdomains and subdirectories are materially identical. Others in the industry agree. It’s a persistent myth that probably has much more to do with voodoo than fact.