||This article needs additional citations for verification. (August 2011)|
nofollow is a value that can be assigned to the
rel attribute of an HTML a element to instruct some search engines that a hyperlink should not influence the link target's ranking in the search engine's index. It is intended to reduce the effectiveness of certain types of internet advertising because their search algorithm depends heavily on the amount of links to a website when determining which websites should be listed in what order in their search results for any given term.
Concept and specification 
nofollow value was originally suggested to stop comment spam in blogs. Believing that comment spam affected the entire blogging community, in early 2005 Google’s Matt Cutts and Blogger’s Jason Shellen proposed the value to address the problem.
The specification for
nofollow is copyrighted 2005-2007 by the authors and subject to a royalty free patent policy, e.g. per the W3C Patent Policy 20040205, and IETF RFC 3667 & RFC 3668. The authors intend to submit this specification to a standards body with a liberal copyright/licensing policy such as the GMPG, IETF, and/or W3C.
<a href="http://www.example.com/" rel="nofollow">Link text</a>
Introduction and support 
Google announced in early 2005 that hyperlinks with
rel="nofollow" would not influence the link target's PageRank. In addition, the Yahoo and Bing search engines also respect this attribute value.
Interpretation by the individual search engines 
While all engines that use the
nofollow value exclude links that use it from their ranking calculation, the details about the exact interpretation of it vary from search engine to search engine.
- Google states that their engine takes "nofollow" literally and does not "follow" the link at all. However, experiments conducted by SEOs show conflicting results. These studies reveal that Google does follow the link, but it does not index the linked-to page, unless it was in Google's index already for other reasons (such as other, non-nofollow links that point to the page).
- Yahoo! follows it, but excludes it from their ranking calculation.
- Bing respects "nofollow" as regards not counting the link in their ranking, but it is not proven whether or not Bing follows the link.
- Ask.com also respects the attribute.
|Uses the link for ranking||No||No||No||?|
|Follows the link||Yes||Yes||?||No|
|Indexes the "linked to" page||No||Yes||No||No|
|Shows the existence of the link||Only for a previously indexed page||Yes||Yes||Yes|
|In results pages for anchor text||Only for a previously indexed page||Yes||Only for a previously indexed page||Yes|
Use by weblog software 
Many weblog software packages mark reader-submitted links this way by default (often with no option to disable it, except for modification of the software's code).
More sophisticated server software could spare the nofollow for links submitted by trusted users like those registered for a long time, on a whitelist, or with an acceptable karma level. Some server software adds
rel="nofollow" to pages that have been recently edited but omits it from stable pages, under the theory that stable pages will have had offending links removed by human editors.
The widely used blogging platform WordPress versions 1.5 and above automatically assign the
nofollow attribute to all user-submitted links (comment data, commenter URI, etc.). However, there are several free plugins available that automatically remove the
nofollow attribute value.
Use on other websites 
||This article's factual accuracy may be compromised due to out-of-date information. (November 2010)|
MediaWiki software, which powers Wikipedia, was equipped with nofollow support soon after initial announcement in 2005. The option was enabled on most Wikipedias. One of the prominent exceptions was the English Wikipedia. Initially, after a discussion, it was decided not to use
rel="nofollow" in articles and to use a URL blacklist instead. In this way, English Wikipedia contributed to the scores of the pages it linked to, and expected editors to link to relevant pages.
In May 2006, a patch to MediaWiki software allowed to enable nofollow selectively in namespaces. This functionality was used on pages that are not considered to be part of the actual encyclopedia, such as discussion pages and resources for editors. Following increasing spam problems and a within-Foundation request from founder Jimmy Wales,
rel="nofollow" was added to article-space links in January 2007. However, the various interwiki templates and shortcuts that link to other Wikimedia Foundation projects and many external wikis such as Wikia are not affected by this policy.
Other websites like Slashdot, with high user participation, add
rel="nofollow" only for potentially misbehaving users. Potential spammers posing as users can be determined through various heuristics like age of registered account and other factors. Slashdot also uses the poster's karma as a determinant in attaching a nofollow tag to user submitted links.
Social bookmarking and photo sharing websites that use the
rel="nofollow" tag for their outgoing links include YouTube and Digg.com (for most links); websites that don't use the
rel="nofollow" tag include Propeller.com (no longer an active website) (formerly Netscape.com), Yahoo! My Web 2.0, and Technorati Favs.
Search engines have attempted to repurpose the nofollow attribute for something different. Google began suggesting the use of
nofollow also as a machine-readable disclosure for paid links, so that these links do not get credit in search engines' results.
The growth of the link buying economy, where companies' entire business models are based on paid links that affect search engine rankings, caused the debate about the use of
nofollow in combination with paid links to move into the center of attention of the search engines, who started to take active steps against link buyers and sellers. This triggered a very strong response from web masters.
Control internal PageRank flow 
Search engine optimization professionals started using the
nofollow attribute to control the flow of PageRank within a website, but Google since corrected this error, and any link with a nofollow attribute decreases the PageRank that the page can pass on. This practice is known as "PageRank sculpting". This is an entirely different use than originally intended.
nofollow was designed to control the flow of PageRank from one website to another. However, some SEOs have suggested that a
nofollow used for an internal link should work just like
nofollow used for external links.
nofollow on internal links pointing to them. Google employee Matt Cutts has provided indirect responses on the subject, but has never publicly endorsed this point of view.
The practice is controversial and has been challenged by some SEO professionals, including Shari Thurow and Adam Audette. Site search proponents have pointed out that visitors do search for these types of pages, so using
nofollow on internal links pointing to them may make it difficult or impossible for visitors to find these pages in site searches powered by major search engines.
Although proponents of use of
nofollow on internal links have cited an inappropriate attribution to Matt Cutts (see Matt's clarifying comment, rebutting the attributed statement) as support for using the technique, Cutts himself never actually endorsed the idea. Several Google employees (including Matt Cutts) have urged Webmasters not to focus on manipulating internal PageRank. Google employee Adam Lasnik has advised webmasters that there are better ways (e.g. click hierarchy) than
nofollow to "sculpt a bit of PageRank", but that it is available and "we're not going to frown upon it".
No reliable data has been published on the effectiveness or potential harm that use of
nofollow on internal links may provide. Unsubstantiated claims have been challenged throughout the debate and some early proponents of the idea have subsequently cautioned people not to view the use of
nofollow on internal links as a silver bullet or quick-success solution.
More general consensus seems to favor the use of
nofollow on internal links pointing to user-controlled pages which may be subjected to spam link practices, including user profile pages, user comments, forum signatures and posts, calendar entries, etc.
Employment of the nofollow attribute by Wikipedia on all external links has been criticized by web authors for not passing the deserved rank to referenced pages which serve as the original source of each Wikipedia article's content. The decision was enacted on Wikipedia to combat spamdexing on its pages, which are an otherwise tempting target for spammers as Wikipedia is a very high ranking site on most search engines. The drawback for original publishers is that not only must they compete with the Wikipedia article for a higher rank in search results, their website does not receive the increase in rank that otherwise would have been contributed without nofollow.
Use of nofollow where comments or other user content is posted (e.g. Wikipedia) not only depreciates the links of spammers but also of users that might be constructively contributing to a discussion, and preventing such legitimate links from influencing the page ranking of the websites they target.
See also 
- Google PageRank
- Search engine optimization
- Search engine spiders, also called web crawlers
- Spam in blogs about nofollow
Blocking and excluding content from search engines 
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