“Subdomain or Subdirectory Name for a SEO Blog?–It depends!“
Some quick definitions of “Subdomain” and “Subdirectory”:
A subdomain is a completely different website (from the ‘main-domain’) and it looks like this, “http://subdomain.main-domain.com”.
*Note–For ease of clarification, I’ve used ‘main-domain’ instead of ’site-domain’ as the Top Level Domain (TLD): e.g. main-domain.com or Top-Level-Domain.com
The URL of a subdirectory looks like this: “http://main-domain.com/subdirectory.”
Search engines see subdirectory just that–subdirectory of the website’s domain. Basically, what that means is that the subdirectory belong to the site’s main domain or subdomain, which again is a different website from the ‘main-domain’.
So, if the main-domain has high page rank (PR), the page rank gets distribute throughout the subdirectories within the main-domain; but the subdomain does NOT, however, feel the PR love from the main-domain 
. This is, again, because the subdomain is a different website from the main-domain.
O.K. Now that we’ve gotten the subdomain/subdirectory definitions down, let’s go back to the question of whether a subdomain name or a subdirectory name is best fitted for your SEO blog.
To simply put, if you have have a main-domain site about cat, “www.cat-domain.com,” and you want to create a blog on “cat food,” then you should choose “cat-food” as a subdirectory for your SEO blog, “www.cat-domain.com/cat-food.” This is because “cat-food” is related to your ‘main’ website topic, so your subdirectory SEO blog will feel ‘more’ PR love from your ‘main-domain’.
On the other hand, if you want to create another site for “dog-food,” you should choose “dog-food” as a subdomain of the “cat-domain.com,” since “dog-food” has little relevance to your main-domain site, www.cat-domain.com.
And because a subdomain is a different website, you can actually have a subdirectory-blog in the subdomain (e.g. subdomain.main-domain.com/subdirectory-blog).
So, if you want an SEO blog out of the subdomain, you can have “luxury-dog-food” as a subdirectory name: “http://dog-food.cat-domain.com/luxury-dog-food”. And yes, if you have high PR for the subdomain “dog-food,” your SEO blog on “luxury-dog-food” also feels the PR love from your SUBDOMAIN, but not from your main “cat-domain.com”.
And that’s concluded my rambling on subdomain vs. subdirectory for an optimized SEO blog 
.
Anyway, to recap, if you have a blog topic (”cat-food”) that’s related to your main-domain website (”http://cat-domain.com”), for SEO purpose, you should choose the subdirectory route: cat-domain.com/cat-food. But if your blog is of different topic than your main-domain website (”dog-food”), you should choose “dog-food” as a subdomain name for your SEO blog: dog-food-blog.cat-domain.com.
O.K. Now, I really am finished rambling about the subdomain and subdirectory dilemma 
.
*For more information on how to optimize your blog, please check out the SEO Blogs category.
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One of the better descriptions on the difference between sub domain/directory I’ve read!
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