Google likes to see link neighbourhoods of respected, authority websites that link to each other.
The higher the link popularity is of those websites the more the link popularity of all those websites increase by linking to each other.
By showing these websites in the search engine result pages (SERP's) Google knows that the chance of the visitors finding the information that they need is very high.
That increases the popularity of Google's search engine which leads to more advertisement sales and income for Google.
Examples of related linking to increase the link popularity of your website:
Internet => Promotion => Search engine optimization (SEO) => Link love => Linkbaiting
Marketing => Internet Marketing => Affiliate Marketing => Affiliate Marketing Programs
Backlinks => Linkbuilding => Link popularity => Link love => Reciprocal link exchange
If a page has a low PageRank getting a reciprocal link from it could still benefit your website.
The PageRank value of a webpage is updated only once a month.
So that webpage could have a much higher PageRank without you knowing that.
The other way round for webpages with a high PageRank.
The formula of PageRank changes just like the Google algorithm changes.
If the amount of links pointing to your website increases at an astronomical speed Google does not trust this.
Especially if your website is new (3-6 months old) Google monitores this closely.
Google suspects that the website uses blackhat (illegal) seo techniques to get masses of links.
Google could give that website a death penalty known as being Googleaxed which means that your site is removed from the search index completely.
By gaining a steady amount of five links a day or 35 links a week Google will trust your website.
After your website is 6 months old you can increase the speed that you gain links to your website.
Paying for links is a blackhat search engine optimization (SEO) technique that is clearly disallowed in the Google guidelines.
Using link domains to put backlinks to your website on is another one.
A linkdomain is a domain registered by the same person as the website that every link on that domain points to.
Linkdomains only have value if it is registered by another serson, hosted on a different server with a different IP-address, hosted by a different webhost.
The internal links in your own webiste are important to increase your Google ranking too.
The internal links have to guide the Google crawler to every internal page of your website.
For your visitors it is very important that they can navigate through your website without thinking about it.
And last and most important, internal links distribute the PageRank to all the pages of your website.
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