Do you want to improve your website’s visibility in search? According to Google Webmaster Guidelines, there are two ways to help the world’s number one search engine find your site. Ask them to crawl it and, “Make sure that any sites that should know about your pages are aware your site is online.”
In other words, you have to get links from relevant sites without violating Google’s link building rules. In this post, we’re going to focus on the classification of links as dofollow and nofollow. Specifically, we’ll explain:
How these classifications affect the SEO value of a backlink, how to identify dofollow vs nofollow backlinks, and what nofollow/dofollow percentages appear in the backlink profiles of pages and root domains ranking at the top of SERPs for competitive search terms.
source