This is a very interesting subject and a lot of people don't know if they should use sub domains when building a site, if this is good or this is bad.

There is no straight answer, because is not that easy, there are a lot of factors which you need to take in considerations when you decide that sub domains are good for your SEO strategy and how search engines will see your site. I hope that the lines bellow will help you make a good decision and get the best results with your site.

The first thing which is long debated is if google treats sub domains as different sites, the answer to this is very simple : NO, google does not treat sub domains as different sites, and this comes from an google engineer (Matt Cutts).

An other thing is if using sub domains will help your site to be indexed better, this answer is also NO, using sub domains will not help you get indexed better.

Now, things that do changes when you use sub domains is page rank, the page rank of your main site will drop but the rank of the sub domains will be higher than without sub domains, so there will be more people going to the sub domains pages and less people going to the main page, than without using sub domains, the idea is that page rank will be shared between all the sub domains and the main domain.

As a conclusion I would say that you should use sub domains only if you know exactly why (for example have a better structure of the page) and if your target is not to get the users to a single page but to more pages inside your site. But if you don't know exactly what to do I suggest to use directories (www.yoursite/directory/page.html).

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