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If I have mysite.com, and blog.mysite.com, do search engines see those as 1 site or 2? Will using subdomains hurt SEO? I have some technical reasons why I can't use mysite.com/blog.

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From Vanessa Fox, an ex-Google employee:

Google is no longer treating subdomains (blog.widgets.com versus widgets.com) independently, instead attaching some association between them. The ranking algorithms have been tweaked so that pages from multiple subdomains have a much higher relevance bar to clear in order to be shown.

It’s not that the “two page limit” now means from any domain and its associated subdomains in total. It’s simply a bit harder than it used to be for multiple subdomains to rank in a set of 10 results. If multiple subdomains are highly relevant for a query, it’s still possible for all of them to rank well.

It's unclear what the "some association" part is -- but the parent and child domain like a.example.com and b.example.com obviously have a relationship of some kind, far beyond what two websites named example-a.com and example-b.com would have.

I don't think any Google penalties can go upstream to the parent, otherwise a lot of web hosting services would quickly find themselves delisted, by this (sensible, IMO) logic:

In this instance I’m talking about the exception given to protect free hosts from penalties, particularly those who give their users subdomains such as Hypermart, Xoom, Wordpress.com, Blogger, Tripod etc. This exemption can’t only cover the popular free hosts otherwise no new freehosts would ever stand a chance. As soon as they got a single spammy user their whole site could get banned and poof goes their legit business.

Likewise algorithmically it can’t cover all free hosts because then the big ones like Wordpress.com and Typepad would all be penalized. On a foresight this would also include profile based social sites such as Myspace and outbound linking social sites such as Delicious.

Therefore penalties can only go downstream to the child subdomains from the parent.

Primary domains can pass a penalty to subdomains. Subdomains can’t pass a penalty to a main domain unless the main domain holds a clear relation to the subdomain.

It's not entirely correct to say that a.example.com and example.com are unrelated (as far as Google is concerned). But it does appear that only penalties, not bonuses, are ever transferred across.

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So if I have myapp.com, and I want to help it get found by getting people to link to my blog, will links to blog.myapp.com help myapp.com get shown in search results? – NotDan Oct 4 '10 at 16:36
@notdan yes of course, links between two sites always help rank -- assuming either site has "more" authority than the other to transfer – Jeff Atwood Oct 4 '10 at 21:53

This is my answer from this duplicate question.

Here what Google's Matt Cutts has to say about sub-domains vs folders

My personal preference on subdomains vs. subdirectories is that I usually prefer the convenience of subdirectories for most of my content. A subdomain can be useful to separate out content that is completely different. Google uses subdomains for distinct products such news.google.com or maps.google.com, for example. If you’re a newer webmaster or SEO, I’d recommend using subdirectories until you start to feel pretty confident with the architecture of your site. At that point, you’ll be better equipped to make the right decision for your own site.

Here is what Rand Fishkin from SEOMoz says about sub-domains vs folders

Subdomains SOMETIMES inherit and pass link/trust/quality/ranking metrics between one another

Subfolders ALWAYS inherit and pass link/trust/quality/ranking metrics across the same subdomain

I tend to use subdomains sparingly. I use folders for most things, but I use subdomains in some cases:

  • When the software that powers it is hosted by a third party. This is common with blogs, forums, press releases, and company job openings. You can point DNS for a subdomain to a third party and have them handle running the software, but still have the content be part of your site.
  • For internationalization. I recommend sub-domains for translated content because it enables you to set up a server in the country that you are targeting. You can do that transparently to your users at any point. It is just a DNS change.
  • When content needs to be hosted by a content delivery network (CDN). This is very common for images of a site.
  • When you don't want the same cookies to be sent. Cases such as different logins for different sections of your site, or when your www cookies are large and you create a different subdomain that won't get them for performance reasons.
  • When you may not want to fully vouch for the content. For example you give each of your users a subdomain and let them have free reign with the content that they host there.
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If you have a reputation management problem and want to occupy multiple page 1 positions for your brand. or You have a Panda problem, and need to move low quality pages off your primary domain.

Outside that, subdomains are a problem. For purposes of authority, Google treats them as separate sites. For purposes of relevance, they do, too. As far as I know the only difference is that, for purposes of trust, Google sometimes passes history to the subdomain. That just means you may not see the same lag to indexation you'd see on a brand-new domain.

But if you're trying to get ranking for a competitive term, a subdomain only splits your efforts. Trust gets you indexed. Authority and relevance get you ranked.

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