Determining the placement of certain pages of ones site can be a challenging task for any webmaster. Many people, especially if you are new to developing websites, are perplexed by the question of whether or not to go the subdomain route (blog.example.com) or the directory route (www.example.com/blog).
The general consensus is that subdomains are usually reserved for pages that aren’t completely associated with the general theme of the site. Google, for example has http://maps.goolge.com, http://books.google.com/ and http://blogsearch.google.com/. From an SEO perspective, it has been said that “link juice” will not necessarily flow from the main domain to pages within a subdomain. This has proven not to be the case; however, as some webmasters have seen pages within their subdomains garner the same link value as their main domain.
The real question, though, is whether there is any kind of duplicate content on the subdomain. There is a trap that some webmasters fall into where, because they haven’t set up their site architecture in a logical manner, that some pages of their subdomain duplicate pages from their main domain. While this will rarely incur some kind of penalty within the search engines, it may prevent the search engines from crawling the most important pages on the website given the “crawler caps” the search engine spiders have in place. Matt Cutts of Google has said that for newer webmasters, the subdirectory structure is the way to go until you are more confident with your site’s architecture. I tend to agree.