Do subdomains help/hurt SEO?

Question

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

Answer 1

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.

Answer 2

2007: 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.

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2012: 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

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2017: Google's John Mueller's webmaster video:

Google web search is fine with using either subdomains or subdirectories.

Making changes to site's URL structure tends to take a bit of time to settle down and search so I recommend picking a set up that you can keep for longer.

Some servers make it easier to set up different parts of a website as sub-directories. That's fine for us this helps us with crawling since we understand that everything's on the same server and can crawl it in a similar way. Sometimes this also makes it easier for users to recognize that these sections are all a part of the same bigger website.

On other servers using sub-directories for different sections like a blog and a shop can be trickier and it's easier to put them on separate subdomains. That also works for us you'll need to verify subdomains separately in search console make any changes to settings and track overall performance per subdomain we do have to learn how to crawl them separately. But for the most part that's just a formality for the first few days.

So in short, use what works best for your setup and think about your longer-term plans when picking one or the other

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2018: Google's John Mueller during office hours:

We see these the same.

I personally try to keep things together as much as possible. If it's the same site, try to put them out in the same site essentially. Use subdomains where things are really kind of slightly different.

There are lots of really strong opinions on this. From my point of view this is something that could go either way. If you have really strong reasons to go one way or the other then obviously that might be what you'd want to watch out for. On the other hand if you're like, "well I don't care either way," then I would just keep it within the same site.

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Personally, 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 or can't be easily hosted on the same server. This is common with blogs, forums, press releases, and company job openings. You can point DNS for a subdomain to the other server or have the third party 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.

Subdomains can be just as good for SEO as subdirectories, as long as Google knows they are part of the same website. To make sure they look like they are the same site to Google:

• Link between the subdomain and the main site in the navigation menu, and visa-versa.
• Put the same copyright statement on the bottom of each
• Link each to the same terms of service, privacy policy, and contact page
• Use the same logo on each
• Use the same look and feel: colors, fonts, and layout
• Add the subdomain to the same account in Google Search Console
• Use the same Google Analytics and adversiting accounts (such as AdSense) on each

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Source: my answer from this duplicate question

Answer 3

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.

Answer 4

Yes. It duplicates your search engine results and can result in lowered search engine rankings.