The majority of blogs allow visitors to add comments to a post. However, the moment this comment feature was invented, spammers started to take advantage of it. The idea is to place a link to your website in your comment. Then when a search engine comes to index the blog, finding the link, your website's PageRank increases. In order to combat this problem, Google invented the nofollow attribute value.
Generally placing external links in your webpage or blog post raises its PageRank. This, however, this only true if the external link points to information related to the topic of your webpage or blog post. If not, the link is just search engine spam, therefore Google will lower your webpage's PageRank.
So, when do you apply the nofollow attribute value? When you use free-to-reprint content, the author of the content usually requires you to include a link at the end of the article. Sometimes the webpage pointed to by the link doesn't exist. Maybe the author moved the webpage, or maybe the author just gave up on the effort related that content. In any case, if the webpage pointed to by the link doesn’t exist, you can still meet your obligation to include the link, while not reducing your webpage's PageRank, by using the nofollow attribute value in the link.
When you want to use free-to-reprint content, or you want to give attribution to a resource, but the content pointed to by the link contains nothing but advertising or paid links, you can include the link, while not reducing your webpage's PageRank, by using the nofollow attribute in the link.
nofollow is a value that you assign to the rel attribute of a link. Shown below is an example of a link containing the rel attribute with a value of nofollow.
The nofollow attribute value is used to communicate that you don't want search engines to index the content pointed to by the link. However, today's search engine business is very competitive, and the Yahoo and Bing search engines would like to knock Google out of the top position. To win in the search engine business you need to succeed at finding what the user is searching for.
For this reason, search engines will follow a link with the nofollow attribute value. However, when they discover that the link is bad, they will not index the webpage pointed to by the link, and more imprtantly, they will not reduce your webpage's PageRank for having the bad link.
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