rel="nofollow" attribute in hyperlinks to prevent comment spam and others

I faced this problem while working on a site that, a client wanted a page of his site not to be crawled and added in google's search result.
Google or other search engines, while crawling a submitted site's / existing site's page for new changes or others, they also follow links in that page and in this way the crawler notices a new page in your site.

But suppose, there is a page in your website you don't want to be crawled. Now, you may be wondering why may someone want that ? Here is an example scenario.


Problem scenario 1 :

In my specific case, the requirement was, the client had a product review module and in every product details page, there were links to "add review" like,

<a href="http://www.client_website.com/store/add_review.php?product_id=23" >Add review </a>

 All the pages with different product id had the almost 99.99% the same contents in it.
And the problem was, it is causing to have like 200 pages in google with different links but the same duplicated contents. This duplicate content issue may have negative impact on the ranking of the site as it may seem that the webmaster is trying to duplicate contents and have more pages listed in search engine.


Problem scenario 2 : 

I got this from here. Some spammers try to make their site more visible to search engine by spamming their site's links to the comment section of different related Blogs and Forums. You may have seen this kind of comments. This kind of SEO techniques are known as black hat techniques and if somehow the search engine understand about that a black hat technique has been used to gain a better search engine position, they even demote a site's ranking.
You may want to prevent these spamming and for doing this, all you have to do is to ensure that the spammers doesn't get benefited by their spams in your site and eventually after trying for some time, they will stop trying.

Solution : 

When search engines see the attribute (rel="nofollow") on hyperlinks, those links won't get any credit when we rank websites in the search results.This isn't a negative vote for the site where the comment was posted; it's just a way to make sure that spammers get no benefit from abusing public areas like blog comments, trackbacks, and referrer lists.

About rel="nofollow"

"Nofollow" provides a way for webmasters to tell search engines "Don't follow this specific link"
or "Don't follow links on this page".

Originally, nofollow attribute appeared in the page-level meta tag, and instructed search engines not to follow / crawl any outgoing links on the page. For example:

        <meta name="robots" content="nofollow" />

When nofollow was used on individual links, preventing robots from following individual links on a page required a great deal of effort (for example, redirecting the link to a URL blocked in robots.txt).
That's why the nofollow attribute value of the rel attribute was created. This gives webmasters more granular control: instead of telling search engines and bots not to follow any links on the page, it lets you easily instruct robots not to crawl a specific link. For example:

        <a href="signin.php" rel="nofollow">sign in</a>


Although, this solution was quite simple It is a very important tag to use by the webmaters. That's why,  I thought to share about this in my blog.

For more details and the source I used, 

• The Official Google Blog
• Google webmaster Central
• And the small video from Google Webmaster Central Channel on YouTube