Traditional SEO thinking suggests subdomains are treated as separate websites from their root domain.
This would suggests that ‘link juice‘ is not passed between them. Practically speaking, this means that placing your blog in a sub-domain does not contribute to your SEO efforts, as would fresh and relavent content added to a sub-folder.
Example of a sub-folder: underscore.co.uk/blog
Example of a sub-domain: blog.underscore.co.uk
This point is reinforced in this Moz guide:
Since search engines keep different metrics for domains than they do subdomains, it is recommended that webmasters place link worthy content like blogs in subfolders rather than subdomains. (i.e. www.example.com/blog/ rather than blog.example.com) The notable exception to this is language specific websites. (i.e. en.example.com for english)
However, there has been research both past and recentt, backed up by Google and Matt Cutts, which suggests that they are treated no differently from each other. There’s also this post which concludes:
Since it’s the value that matters and not the topographical structure I submit that there is absolutely no negative aspect to using subdomains for search engine optimization. The value can flow to them and from them and between them just as easily as to, from, and between subdirectories.
So to conclude, should we be using sub-folders or sub-domains for our content? underscore has done some research of our own and we’ve concluded that for relevant content the best position for a blog for SEO purposes is a sub-folder.
The use of sub-domains should be reserved, not only for the experienced webmaster, but for those who need clearly defined segregations between sections of their website.
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