What is a 301 redirect?
A 301 redirect is an instruction that a specific URL (and its content) has permanently moved from one location to another. Proper 301 redirects are implemented technically at a server level.
For example, if your website was hosted on the domain www.goodwebsite.com and you wanted to change it to www.reallygoodwebsite.com, you would implement a 301 redirect from the old URL to the new one. This would ensure everyone visiting, or crawling, the old domain homepage URL was forwarded onto the new domain. 301 redirects like this would be implemented for every single URL within the original domain.
The number 301 refers to the HTTP status code that is returned when the redirected URL is visited. Search engine bots and web browsers are two examples of agents that read these HTTP status codes.
In the above example, any links that pointed at www.goodwebsite.com now in practice point at www.reallygoodwebsite.com for any user ‘physically’ following a link. In terms of link equity, and value, the same is also true. Links which point to your website from another domain via a server redirect still pass PageRank.
Whilst some of the authority, in the eyes of search engines, of the original links is lost, it is important that you view these links as a direct backlink to the new domain, as this is how Google views them.
Within the above understanding in mind it becomes important that when reviewing a link profile to gather link data pointing at the original 301 redirected URL/domain. Failure to do so means that potentially a whole host of links which make up the link profile of the domain being examined are not being taken into account.
Manipulative 301 redirects
A common trick amongst the SEO industry at one time would be to 301 redirect a URL/domain with the sole purpose of passing along authority into the destination URL/domain.
The 301 redirected domains would often be dropped domains that where purchased solely for this purpose. These domains usually held no relevance to the destination domain, nor were these domains ever home to the content or organisation now present on the destination domain.
Often a number of differing domains would 301 redirected into one destination domain. Clearly, such a practice is highly manipulative. In such a case, the original 301 redirect should ideally be removed and the 301 redirected domain disavowed.
There is an instance where a 301 redirect does not pass Page Rank. This method is most commonly used by PPC (pay-per-click) advertising platforms to prevent a link from passing PageRank. In such instances the link is redirected to an intermediate page which is blocked from search engine crawlers with a robots.txt file. PPC platforms which use this method are not limited to search engines.