This article SEO Subdomain looks in to a question -
Are subdomains with diverse topics harmless to the main domain in sense of the search engine optimiation?
Do subdomains affect main domain ranking? Someone says in 2009 that in Googele Webmaster Tools, under “What Googlebot sees” for his main site profile he sees keywords that are mentioned in the subdomains. I checked over keywords and SERPs for tuljo.com in 2011, even as the main domain and subdomains are cross-linked, no keywords of subdomains associate with the main domain.
The interlinking is the key. About.com uses their subdomains like directories and interlinks all over the place, blogspot.com creates a link to the main domain on all their subdomains, a million of which are nothing but spam blogs, but they sure don’t link back to them. 2009.
The subdomain is quite separate from the main domain, so long as there are no links between them. So the nature of the content is irrelevant. 2010.
Subdomains on blogspot.com and hp.com are very different cases.
Should I keep a not well performing domain or move it to a subdomain, as I’d love to keep it’s content for my portfolio in future?
The SEO community has suggested several theories on the topic of SEO Domain in the past. Google handles subdomains and the main domain as different domains, thus example.com, www.example.com and my.example.com are different domains, in SEO means at least.
What are opinions of SEO people today?
Some of the question is answered in an article Multiple Domains vs Subdomains vs Folders in SEO on 27 Sep 2010.
Based on data from SEO tools there are strong indications that a part of Authority and Trust passes to the subdomains only when the domain has a small number of subdomains and when the link structure of the main website passes enough link juice to them. An additional factor can be whether the subdomain keeps the same website and link structure as the main website. On the contrary, when the number of subdomains is too great (ex: blogspot.com) and when there are no links pointing to them from the main domain, then no authority or trust is inherited.
According to other theories, the subdomains are handled like different domains and thus none of those metrics pass to them. The interlinking is the key definitely.
Each subdomain needs as much work and attention as the main one. Do we have enough content and resources to support all these websites? Inbound links vs link baiting. Subdomains for brand building.
Assumably topics of subdomains does not affect SEO of the main domain, anchor text and link relevance works as usually.
However, numerous keyword and SEO tools on the web - that claim that their results are based on Alexa, Google, Yahoo, etc. - show you porridge cooked up of all subdomains and the main domain. Maybe crappy tools? How intelligent is a SE robot?
Some thoughts about the relations of SEO and subdomains and what influence it might have to search engine optimisation.