There are multiple reasons to redirect URLs. For one, your web pages may have moved but their old URLs may still live in users' bookmarks or in search engine indexes. Without implementing some sort or redirection, that traffic would be lost to a 404 Error Page.
On occasions, you may also want to register several extensions for your domain name: 'mydomain.com', 'mydomain.net' and 'mydomain.org', and have 'mydomain.net' and 'mydomain.org' automatically redirect visitors to your site, hosted under 'mydomain.com'.
Furthermore, if your company sells several products, you may want to give each of them an individual domain name, and have it point to a specific subdirectory of your main site. For example, if you own a site called 'businessvideos.com' that sells a product called 'Marketing Made Easy', you may want to set up a domain such as 'marketingmadeeasy.com', and redirect it to subdirectory: www.businessvideos.com/marketingmadeeasy/.
There are several ways to redirect domains, however, most of them will get you in trouble with the search engines. The search engine friendly way to redirect URLs is to use what is know as a 301 redirect (you can see how Google and Yahoo! specifically endorse this kind of redirection). Here is my take about the different redirection methods and their implications on search engine optimization:
Meta-Refresh Javascript Redirect
You can redirect visitors by placing a snippet of javascript code within the HTML code of the page you want to redirect. With this method, you can specify the number of seconds before the visitor is automatically redirected to the new page. Search engines don't like this method, because of the potential for abuse: you could write an optimized page for a non-competitive search term, and then automatically redirect your unsuspecting visitor to whatever URL you want. For example, it could be relatively easy to write a page about english literature, have it indexed and highly ranked by the search engines, and then redirect your visitor to a casino or Viagra site. If search engines allowed this, users would quickly stop trusting them. That is why search engines penalize this practice, and why you should avoid it.
Parked Domains
You could register an additional domain name, park it, and make it point to the DNS servers of your main site's hosting account, so that when somebody types the additional domain, they will be transported to your main site. However, this approach may lead to search engines listing the same content twice, one for your main domain, and one for your additional domain. In the past, unscrupulous webmasters would use multiple domains to spam search engines and directories, making them list the same pages hundreds of times under different domains. Even if your intentions are good, we don't recommend this approach to redirecting your additional domains, since search engines may penalize your site for duplicate content.
302 and 301 Redirects
When a request for a page or URL is made by a browser, agent or spider, the web server where the page is hosted checks a file called '.htaccess'. This file contains instructions on how to handle specific requests and also plays a key role in security. The '.htaccess' file can be modified so that it instructs browsers, agents or spiders that the page has either temporarily moved (302 redirect) or permanently moved (301 redirect). It is usually possible to implement this redirect without messing with the '.htaccess' file directly, using your web host's control panel instead.
>From a search engine perspective, 301 redirects are the only acceptable way to redirect URLs. In the case of moved pages, search engines will index only the new URL, but will transfer link popularity from the old URL to the new one so that search engine rankings are not affected. The same behavior occurs when additional domains are set to point to the main domain through a 301 redirect.
The URL Forwarding Feature
Most domain registrars offer a feature called URL Forwarding. With this feature, you can register a new domain, such as 'mydomain.net', and have it point to mydomain.com (or to any other URL). The problem, however, is that registrars usually do this by implementing a 302 redirect (page moved temporarily). While Google handles 302 redirects very well, passing link popularity from the additional domain to the main one, other search engines don't do this well, diluting link popularity by splitting it between the two domains, and negatively affecting rankings. Therefore, it is better not to use this method, and implement a 301 redirect instead.
Redirecting Old URLs
To '301 redirect' an old URL to a new one, just go to your web host's control panel, and choose the "Redirects" option. You can then set up the redirect by filling the blanks. You want to chose redirect option "Permanent" to implement a 301 redirect.
Redirecting additional domains
To 301 redirect an additional domain (like in the case of the .net or the .org version of your domain name), you have to set it up as an add-on domain with your web host (some hosts offer this option for free, and some others charge a small monthly fee per domain). If the additional domain was not registered with your web host, you will first have to go to your domain registrar and change the DNS (domain name servers) to the DNS of your web host (you may have to wait a couple of days before this change becomes functional). Once you've done this, go to your web host's control panel, choose the "Add On Domains" option, and set up your add on domain as follows:
New Domain Name: additionaldomain.com (Do not put any http:// or www)
Username/directory/subdomain: additionaldomain (Enter 'additionaldomain' by itself. Do not put any '.com' or 'www')
Password: 123ABC (Enter whatever password you want).
Then, set up the redirection by filling the appropriate box with the URL of the landing page (where you want your traffic to go).
Once your additional domain is redirecting to your landing page, take this one last step to see if everything is working fine: go to a server header checking tool, type your add-on domain in the query box and hit enter. If you get a message similar to this: "Status Code HTTP/1.1 301 Moved Permanently", then your 301 redirect is working.
You can also use 301 redirection for common mispelled versions of your domain name, or for other good domain names that you don't want your competitors to get.

About The Author

Mario Sanchez publishes The Internet Digest ( http://www.theinternetdigest.net ), an internet marketing content site packed with useful articles and resources, and SEO Tutorial (http://www.seotutorial.info) where you can learn the basics of search engine optimization in four easy steps.