Google Classifies Subdomains as Internal Links
Google announced an important change to how Google Webmaster Tools (GWT) classifies subdomains.
The search engine previously classified subdomains as a separate site entirely - meaning that a link from a subdomain was considered an external link within Webmaster Tools. As you might imagine, many SEO's have been aware of the positive influence of subdomains on primary site rankings for many years.
Google has switched it up on Webmasters however, changing it so that GWT now considers links from subdomains as internal links.
So if you have subdomains on your website those subdomains now appear within GWT as links coming from within your own site and not as links coming from other sites. If you benefitted heavily from subdomain links to your primary site in the past, the good times may now be coming to an end.
While the total impact is not yet known, and we're unsure wether Google will (or already has) modified its algorithm to deprecate the value of links from subdomains, it would be wise to expect some shifts in the interim on the search results.