SEO

Towards the tail end of PubCon, I noticed a spate of articles talking about subdomains and subdirectories in Google, and I wanted to talk more about this subject in case I was unclear.

Historically, it’s been kind of a wash about when to use subdomains vs. subdirectories. Just as a reminder, in a URL such as subdomain.example.com/subdirectory/ , the subdomain is “subdomain” and the subdirectory is “subdirectory” (also sometimes called a folder). If you’re still unclear, you may want to read my tutorial on the parts of a URL.

If you throw your content into a subdirectory, usually the code is all in the same file storage space. That can make it much easier to find/edit/change code. It can also be easier to move code from one place to another. A subdomain, however, is often a domain name system (DNS) alias. Subdomains can be a little more difficult for a novice webmaster to set up, especially if words like “CNAME” don’t mean anything to you. Subdomains can be managed separately, which can be a joy (you can use DNS to decouple a subdomain if you want to migrate that part of your site) or a pain (it may be more of a hassle to juggle DNS setting instead of just using file commands to make or move directories). When I started my blog, I decided to use a subdirectory (mattcutts.com/blog/) just to keep things simple, for example.

For several years Google has used something called “host crowding,” which means that Google will show up to two results from each hostname/subdomain of a domain name. That approach works very well to show 1-2 results from a subdomain, but we did hear complaints that for some types of searches (e.g. esoteric or long-tail searches), Google could return a search page with lots of results all from one domain. In the last few weeks we changed our algorithms to make that less likely to happen in the future.

This change doesn’t apply across the board; if a particular domain is really relevant, we may still return several results from that domain. For example, with a search query like [ibm] the user probably likes/wants to see several results from IBM United States. Note that this is a pretty subtle change, and it doesn’t affect a majority of our queries. In fact, this change has been live for a couple weeks or so now and no one noticed. The only reason I talked about the subject at PubCon at all was because someone asked for my advice on subdomains vs. subdirectories.

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.

Update: Made it more clear that this change has already been live for a while.

Source: Subdomains and subdirectories

It's pretty much common practice to use subdomains for different content and subfolders for similar content. Cutts, I believe, at some point also mentioned larger sites rank better - with that in mind subfolders sound even better so long as the content is related.

I believe subdomains are great if you want to make a 'mini site' within your domain to start up a completely different functionality that need its 'own space' to really stand out from the rest of the site.

A blog or profiles, wiki, forum, gallery, site, shop, free mail etc. could all have different (sub)domains.

I've used a subdomains for the mentioned reasons and to just make the url a little cleaner to look at.

Really interesting inputs on sub-domains.

As far as search engines are concerned, they give less priority to sub-domains ( if compared to original .com or .net domain)