Go to your web browser and key in http:yourdomainname. Now, go to http:www.yourdomainname (with the www). Did your URL change to one or the other or did you see both the www and non-www versions? If it didn’t, you could be splitting your PageRank in half – causing it to be diluted and lowering your rankings. You can solve this problem by setting a preferred domain in Google Webmaster Tools. Here’s how.
How to Set Preferred Domain
1) Verify Both Website Versions in Google Webmaster Tools
Let’s say that you have already verified your website (we’ll assume you verified the ‘www’ version) in Google Webmaster Tools. Now, verify the other version (non-www) as well.
If you’re not sure how, here’s a link.
2) Click on the Gear Button
You’ll find the Gear button on the top right-hand corner.
3) Click ‘Site Settings’
A dropdown menu will appear. Click on the option marked ‘Site Settings’. No screenshot here, sorry.
4) Now, Set Your Preferred Domain
Choose which domain you would prefer to be shown. Google will then consolidate links to both domains to the one you prefer. This simple fix helps prevent your PageRank being split among the different versions of your website.
Remember, you’ll need to have verified both versions of your website before you can actuate the change.
If your website isn’t new, Google will take a while to detect your preferred domain so you might have to wait a week or two to see any effects.