http://www.domain.com vs. http://domain.com

I would like to keep all posts and pages I create with WordPress in the “http://www.domain.com/blog/post-name” format.

However, when I click on a post title, it automatically goes to “http://domain.com/blog/post-title” without showing the “www.”

Is there an easy way to make all posts and pages show the “www?”

My thanks for your help in advance.