Search results may produce thousands of entries spread over hundreds of pages. There is no point appearing further down the list than the second or third page of results because few Web surfers venture further. Ideally, a Web page needs an entry on the first page of results, which means the top 10. Getting into the top three puts the Web page within the initial view of the results and does not even require the user to scroll down before it is seen. The order of results in a search is called the ranking and SEO aims to give a page the highest possible ranking.
Search engines like Google and Bing are an important source of traffic for websites. What's more, they are free. Search engine optimization is the application of design principles, keywords, tips and outright cheats to try to get a website listed as high up in search results as possible. Whether subdomains are good or bad for SEO has changed over the years and is still open for debate.
Even better than being one of the first websites in the list on the first page is being the only website in the list on the first page. Subdomains were used for this strategy. Search engines try to prevent one website from having more than two entries in the search results. This is called “crowding." However, that count was made per subdomain, so a popular SEO trick was to create several subdomains on a site and work to get two pages from each in the top results, thus blocking out the competition.
Any SEO trick has a very brief shelf life, because plenty of other website owners are combing the Web for SEO tips. When everyone uses subdomains to create crowding, search results get clogged up with repetitive results and no one manages to completely crowd out anyone else.
Search engine engineers became annoyed by the repetition of sites and so started to stress the quality of a site in their ranking algorithms. A page that copied the content of other pages got demoted, or worse, removed. In some instances, all the pages that had similar content were removed and so the subdomain trick made websites worse off than if they had never done any SEO. Since 2007 Google has treated subdomains the same as subdirectories of a site and includes them in the appearance count that limits the sites entries in the results. So, subdomains have no influence at all on Google rankings. However, subdomains that repeat the content of other pages on the same site, or other domains, will damage the site's ranking.
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