The HTTP 301 status code indicates that a new permanent URL has been assigned to the target resource and that any future links to this resource should use the updated suggested URLs.
Google Officially on 301 Redirects
When to use 301 redirects? According to the instructions of Google Webmaster Tools, 301 redirects should be applied in the following instances:
- Changing a domain or domain name. If users know your brand and direct traffic is high enough, you will avoid confusion with a new address.
- Some sections are duplicated at several different URLs, for example, http://example.com/home, http://home.example.com and http://www.example.com.
- You are updating an old site or migrating to a new site. When users go to outdated pages, they must be redirected to the newest version.
When you use a 301, Google removes the old page from their index and most value (link equity) from that page is transferred to the new one.