Michelle Blanc

C’est une question qui m’est souvent posé et à laquelle, le porte-parole officieux de Google, Matt Cuts, répond. En gros, si vous avez un sous-sujet vraiment important à ajouter à votre site, un sous-domaine est indiqué. Par exemple, vous êtes un hôtelier qui avez aussi un golf et un spa à promouvoir, un sous-domaine pourrait être approprié dans votre cas. Si votre hôtel s’appelle ABC, vous auriez ABC.com et golf.abc.com et spa.abc.com au lieu de abc.com/golf. Cependant, vos sous-domaines devraient être vraiment exhaustifs sinon, un sous-répertoire fera l’affaire.

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.
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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.

MAJ 

Selon le copain Baillargeon, un sous-répertoire est toujours mieux qu’un sous-domaine étant donné les changements d’algorithmes de Google. Il semble que les sous-domaines ne soient plus ajoutés comme pages vues au domaine. Donc, oubliez les sous-domaines complètement et faites plutôt des sous-répertoires comme abc.com/golf ou abc.com/spa…

MAJ2
Selon SearchEngineWatch (pointé par Baillargeon)

1. From a ranking perspective it truly makes no difference. It's all in the links (had to get the title in there somehow). If you have a subdomain (content.yourdomain.com) that is linked to by the main domain (www.yourdomain.com), and you also have a subdirectory (www.youdomain.com/content) that is linked to by your domain in exactly the same way, the crawler treatment, and the indexing and ranking of the two will basically be identical.
2. That said, I do think that there is a slightly greater risk that a subdomain will be treated as a separate site by Google. While this risk is fairly low, I don't think that there is zero risk of this. I say this because it is entirely conceivable that the subdomain is not be operated and managed by the same people who operate and manage the main domain.
This risk is probably greater for smaller sites that are not yet trusted as much by Google. Bottom line is that if you have a relatively new site, I would steer away from subdomains and stick with subfolders. No need to complicate your life after all.
3. For larger more well established sites, I don't think there is any risk of this being an issue, and subdomains or subfolders is a matter of choice. I've worked with massive media companies that have large families of large sites, and that use subdomains heavily.
On these types of sites, subdomains can provide a preferable way to organize the site, particularly when dealing with highly varied topics or channels of delivery (e.g. video, mobile, etc.). This ie especially true if the nature of that content, or the nature of the channels, is such that people are likely to link to it directly.