When you register a domain and integrate with hosting then you would see you can access in your site by using domain in 2 ways, like, www or non-www (e.g. http://www.bloggerspice.com or http://bloggerspice.com). And those are known as preferred domain. Most of us don’t know why www or non-www versions of the URL both work together. Though it hasn’t any significant value to us.
But this is very important in the eye of search engine. Preferred domain used by search engine to index a site’s pages. I have received email from some new blogger with the non-index issue. Their main problem is that their site’s page is not indexing by Google webmaster. In this case you have to check that which version of domain version is indexing by Google.
If you see non-www version (e.g. http://yourdomain.com) of URL is not indexing then you have to check that www version (e.g. http://www.yourdomain.com) are indexing by Google or not? Because Google index one version of domain which you preferred for search result.
You can easily preferred a domain www or non-www versions from Google Webmaster tools. That means you can choose whether your blog or website link will display with www or non-www versions of the URL.
For bringing changes you may need to verify your blog ownership of both the www and non-www versions of your domain. Because the new setting of preferred domain will impacts on both crawling and indexing. But you have already verified your domain once then it may not require. But if you've removed the previous verified file, Meta tag, or DNS record, you'll need to repeat the verification steps.
Anyway Just follow the below steps for specification of preferred domain.
Step 1 Go to Google Webmaster Tools and Sign in to account
Step 2 Now click on any site (If you have more than 1 blog site).
Step 3 Click the Gear icon and then click on Site Settings.
Step 4 In the preferred domain section select Display URLs as www or Display URLs as non-www
Step 5 Finally hit the save button.
After successfully choosing preferred domain it will take certain period of time to see the change on search engine.
If you don’t specify the preferred domain www or non-www version then by default Google may treat the www and non-www versions of the domain as separate references to separate pages.
There are a bad impact on rapid specification of preferred domain that if you constantly change preferred domain then Google search engine will index both www or non-www versions of the URL. As a result site link may index same content link 2 times that have a bad impact on search engine.