“WWW vs Non-WWW” – How you set up your site makes a difference to the search engines
Here’s what I mean. If your domain isn’t set up correctly from the get go, you are absolutely sabotaging your potential success and search engine rankings – because people are stupid or just lazy.
301 redirect 1 Is Your Domain 301 Redirect Set Up Correctly?
When a web site is set up on a web host, the web host or the domain owner needs to set up a 301 redirect for new site / domain so that the non-www. url points to the www version of the domain OR visa versa. 
Many hosts do this automatically. However, WordPress for example often installs without the “www” by default.
What this means is that IF you go directly to www.domain.com AND domain.com (no www) then the site is not set up correctly on the server, and your rankings will suffer for it.
Here’s why – Google will come and spider BOTH versions of the domain, because it’s sees TWO unique sites addresses.
The problem is that both sites contain the same content so you get whacked for duplicate content.
Now some people will say that there is no such thing as a “duplicate content penalty” and they validate their argument by pointing out the curation model used by news aggregation sites like Mashable.com. And they’d be right at least in part.
There is no duplicate content penalty on the web. However there IS “on-site”. What makes this worse is that the www and the non-www versions of your site that Google sees are EXACT copies and that will cause a problem.
Interestingly though that’s not your biggest issue.
The real issue as I see it is “Link Dilution.”
The term “Link dilution” is where you have other sites linking to you naturally because they like your site, they think you offer good value or because they’re talking about you or a comment you’ve made on your blog for example.
Now that doesn’t concern us really, what does though is HOW other people are linking to those sites.
Some people might link directly to the “www version” and some might link to the non “www version” of the site.
In other words, since people are sending link juice to both sites, neither site is getting all the link love it should be getting.
What this does when combined with Google spidering both versions (they see 2 sites remember) is that Google generates KW rankings for EACH site.
The problem is easy to fix though.
1) CHOOSE which version of your site you want to use – with our without the WWW.
2) Make sure that your web hosts uses a 301 redirect on the version you are NOT using, and have that unused version of your domain point TO the version of your site that you’ve chosen in Step #1.
In other words if you decide that default for your site is going to use the www prefex (http://www.yourdomain.com) then you want to go into your online web hosting control panel and set up a 301 redirect for the non-www version of the domain so that it points to the www.version.
Here’s how to check if this redirect is already set up on your existing site.
(We’re going to assume the domain is set up to use “www” as the default in this case)
Go to your web browser and type in “http://yourdoman.com”
Now if your web hosting account is set up correctly, what you’ll see appear in your browser window is the http://www.yourdomain.com version of the site.
If however, the redirect doesn’t happen you should set one up ASAP. It’s not difficult to do – especially if your web host uses CPanel – you can always just call your hosting provider and have them set it up for you. It takes less then 2 minutes.