What is a Sub Domain?
You know that www. bit at the beginning of websites? As in, WWW.noisylittlemonkey.com that "www." is a subdomain of noisylittlemonkey. Using a www. subdomain has become a common convention when structuring web addresses. It isn’t necessary to have a subdomain at all, but it may be helpful if you want to organise various separate resources on a single “domain” (read: Second Level Domain and Top Level Domain combo). Some businesses opt to have "shop." Instead of "www." to separate out their shop from the rest of the website, similarly some people use "news." or "blog." to separate out these resources. Sometimes you’ll see organisations using "UK." or "USA." or "FR." instead of "www." to separate their country specific sites. Technically, you can have as many subdomains as you like, but one is usually sufficient.
What’s important for Search Engines?
Search engines understand that blog.yourdomain.com and www.yourdomain.com are related. However, in practice, they tend to treat them as seperate entities altogether. Our advice is to get everything living on one subdomain. Normally the "www." version. Put country specific sites, shops, blogs, news, everything in sub folders. Why? Because when a trusted site (say the BBC) links to the "www." version of your website, if everything else on your website lives on that SUB Domain, it will all inherit a little trust / authority from that link. If bits of your website are living in different sub domains (e.g. "shop." and "blog." ) these won’t inherit any* trust from that link and therefore won’t see a boost in the search engine results pages as a result.
(*there is minimal sharing between subdomains, otherwise they're treated as unique entities.)
What’s important for everyone else?
People are used to "www.", so we tend to recommend that our clients stick with it. Your prospective customers will probably Google you anyway.
A typical web address for an organisation is comprised of three elements. Read more about how you can use them to get better search engine rankings by following the links:
For more on URLs, SEO, UX and all manner of web address related acronyms, have a gander at our Anatomy of a URL post.