Determining whether to use www in your canonical URLs is an important decision and should be reinforced by redirects to prevent duplicate content from an SEO perspective.
Definition of a WWW Redirect
A www redirect is a rule on your web server that forwards all traffic from the non-www version of your domain to the www version, or vice versa.
For example, lets say your web address is www.example.com and someone types in example.com into their browser. The browser will send them to example.com. When that request hits your web server, the redirect will tell the browser that the correct web address is actually www.example.com and will send the user there instead. You can also setup redirects that work the other way around and send users from www.example.com to example.com.
If you are trying to decide whether to use a naked domain or a www subdomain in your canonical URLs, be sure to read the article “Why use WW?“.
The HTTP protocol used on the web has a numerical system for identifying the status of page requests. The 301 status code indicates that the page or resource requested has ‘moved permanently’. When implementing your www redirect it is important that you return a 301 status code so that search engines are clear which URL should be indexed.
Avoiding Duplicate Content with Redirects
Every website has what is called the ‘site root’. That is the main directory on your web host that is publicly available when a user goes to your domain. By default, this directory can be accessed by typing in the root domain one of two different ways www.example.com or example.com. This means that for every page on your website, it can be accessed via two different URLs: the www version and the non-www version. When search engines, such as Google, come along to crawl and index your site, they may come to the same page via the two different versions of the URL. When a search engine finds the same content at different URLs, it is called duplicate content.
Duplicate content is an issue because it puts your own site in competition with itself. A page that might normally have great rankings in search now has another copy of itself in the search index. When a user types in a keyword, how does the search engine know which page to display? Is one copy better than the other? To find the answer, search engines will look at off-page factors such as the number of incoming links. If you have links pointing to just one of the pages, then that one will probably be selected. If not, then that means you have users linking to two different URLs for the same content. Imagine how much better you would rank in search if all those links were for the same page. Allowing the same content to be accessed via multiple URLs dilutes your SEO efforts and devalues your content in the eyes of search engines. The fix is to simply redirect everyone, search engines included, to the desired URL using 301 redirects.
Locating and working with your .htaccess file
If you aren’t using an Apache server, then what I am about to tell you won’t work. If you are, then you will need to find or create an .htaccess file at your site’s root directory.
The .htaccess file may, or may not, be visible when viewing your site’s file structure through cPanel or your favorite FTP editor. If you don’t see an .htaccess file, look around for an option that will allow you to see hidden files. If you still don’t see it, then go ahead and create a new file named .htaccess. If you have to create one, you may have to upload an empty text file and rename it to .htaccess (not .htaccess.txt). Once you have found or created your .htaccess file, then you just need to place your redirect rules within it.
If your .htaccess file already has stuff in it, make a copy and save it somewhere in case you have to revert back! There is nothing worse than breaking your entire site because you messed up an important file and didn’t save a backup.
Implementing the WWW Redirect
First, make sure that mod_rewrite is enabled. Just make sure that this line appears somewhere in your .htaccess file above the rule that we are about to add:
RewriteEngine On
To redirect from non-www to the www version of your site, use the following rule:
RewriteCond %{HTTP_HOST} ^example.com [NC]
RewriteRule (.*)$ http://www.example.com/$1 [R=301,L]
To redirect from www to the non-www version of your site, use the following rule:
RewriteCond %{HTTP_HOST} ^www.example.com [NC]
RewriteRule (.*)$ http://example.com/$1 [R=301,L]
Make sure that you replace example.com with your real domain!