Original article credit: Special thanks to Melissa J Luther
Using multiple domain names to boost SEO value of one Web site is a tricky beast. While it might seem simple enough to register many domains containing keywords for which you’d like to rank and then routing traffic through them to your primary Web site, it is actually not so easy.
For one thing, search engines do not like duplicate content, and that is what multiple URLs pointing to the same content looks like to them. You have no doubt heard of the “Google duplicate content penalty.” This is not actually a penalty actively imposed by Google—or any other search engine for that matter. Rather, it results from the desire of search engines to provide users with a variety of relevant content.
If a search engine determines that many URLs serve up identical content, the engine will select a preferred version (often referred to as the “canonical” version) and reduce the value of the other copies. It will do this for each page for which it finds apparent duplicates, and the canonical version may not be the same domain name for each page. This can cause the search engines to consider your internal linking inadequate and ultimately devalue all your domains.
It is also just plain easier to promote only one site. Inbound links are a major contributor to how search engines view a site. With one domain, all links point directly to that domain and it’s simple for search engines to calculate the inbound link values.
Using multiple domains to promote one Web site dilutes your inbound link values. If you have two URLs with half of your inbound links pointing to your primary domain and the other half to a secondary domain, each URL gets credit for only half of the links it could have had if you were using only one domain.
The Only Safe Way: 301 Redirects
The only safe way to promote multiple domain names pointing to one Web site is to 301-redirect all the secondary domains to the primary domain. Search engines recognize the 301 redirect as a valid method of permanently rerouting traffic from one domain name to another. The search engines will also pass link value through a 301 redirect so that your primary domain gets credit for links to any of your secondary domains.
Great information for multiple domain redirection here.
If you consider multiple domain names as vital to your branding and traffic strategy, then make sure your primary domain gets the credit it deserves and use 301 redirects to point all visitors and search engines robots in the right direction. You can promote keyword-rich, easy-to-remember domain names and still get traffic to your primary domain Web site.