yourdomainname.com
www.yourdomainname.com
yourdomainname.com/index.php
yourdomainname.com/default.asp and etc.

All of these URLs can be considered different URLs in the search engines which can mean trouble in the SERPs for your site. If people place links to each of these URLs then you are in essence building a link campaign for different URLs. By setting up a 301 redirect these inbound links to ONE centralized URL. Many SEOs choose the www. version to redirect all links to as its more user friendly.

What are some reasons you might want to move a URL permanently?

1) If you have designed a new page on your site and are not using the old page. You would want to do a 301 redirect so anyone who has linked to this (now dead) page will be redirected.

Example: Your new page is http://www.yourdomain.com/seo.html and you are putting this up to replace http://www.yourdomain.com/searchengineoptimization.html

When moving or renaming pages it is important to also consider people who have bookmarked your site. Setting up a 301 will be redirect them to your new page. If you do not do a 301 redirect then you will want to make sure you have an optimized 404 page.

2) In order to redirect link credit to a new page then a 301 redirect will need to be placed on that page.

3) If you are looking to forward your pages PageRank to a new page then a 301 redirect is necessary.

4) If you have canonical issues and would like to resolve misplaced inbound links to different versions of your domain name then you will want to use a 301 redirect.

What is the most common use for a 301 redirect?

Canonical issues are vary common and usually overlooked when doing SEO but this issue has been known to cause large problems in link development and general SEO ranking. I was recently checking out webmasterworld.com and I found a great post on canonical problems and forward discussions on this problem.

I have been setting up 301 redirects for some time now and I have been fixing canonical problems for years. If anyone is interested in some resources on fixing these issues then check out webconfs.com for 301 redirects across multi platforms and tamingthebeast.com for some great walk throughs on 301 redirects. Usually modifying your .htaccess file or setting up a PHP redirect can fix your problem. If you run into problems you might consider contacting your web host for further help.

Remember, your inbound linking structure could be much more effective if it was not diluted by varying URLs caused by canonical problems. If a search engine indexes all versions of this URL then you could fall into a duplicate content filter as well. It is best to set up a 301 redirect to help avoid problems in the future or quickly fix problems that you have already noticed.