Does a sub-domain benefit a domain...

[Switch_Digital]

I have seen some mixed comment as the whether a sub-domain would benefit from the authority built up by its domain... but does a domain benefit from a sub-domain?

[seomozinator]

From everything I have read, a sub domain is treated as an entirely different website.  So there is no good reason, as far as seo, to have a subdomain unless you simply want to separate your site info.  However, keep in mind that all search data is separate.

[BeanstalkIM]

Back in 2007 Matt Cutts announced at Pubcon that sub-domains and folders were to be treated the same.  Of course, that didn't exactly happen as it was supposed to but the principle is sound.  If what you're worried about is how weight is passing then it's pretty much a level field where you're left to chose the one that makes the most sense for your backend.  I personally tend to lean on sub-directories whenever possible for predictability and simplicity.

I will note that I have quite recently seen penalties against a sub-domain on a large-scale site affect the rankings on the domain as a whole so there's definitely crossover in domain authority and trust.

The quickest answer to your question is, yes the domain will benefit from the sub-domain.  But it's probably still easier to use a folder.

[Everett]

I have heard conflicting things from people at Google, including Matt Cutts, throughout the years. I have also seen some first-hand anecdotal evidence that subdomains inherit some benefit from the parent domain, though much less evidence that parent domains gain anything from the subdomains.

It is my opinion that the devil is in the details here. If, for instance, you have a site with dramatically different topics on various subdomains, as if they were completely different websites, I think it is more likely that Google will treat them as such. However, if you have a site with a subdomain for a certain section, such as a forum or blog that is of the same, or very similar topic, with lots of interlinking between the two - as if they were different parts of the same site - I think it is more likely that Google will treat them as such.

So I don't think the answer is as cut-and-dry as people often like to think, which is why it is so difficult to find consistent proof of one theory or the other.

In the end, do what's best for your users and developers. Most of the time that means a subdirectory, but every once in awhile it means a subdomain.

I'll leave this open for other opinions. Maybe someone has some empirical proof that we can look at.