Are you referring to backlinks? Yes quality backlinks from reputable sites are an important part of the SEO equation.

Yes internal links are important assuming they're relevant pages (silo) and you also have a decent followed external backlink profile.

Internal links are extremely important, through the links and habits clicking these links by Google users will know which site you're talking about what topics thus be considered Internal Link 1 the factors most important SEO. You try to create internal links in your site offline.

MozBar help see the relevant data when you surf the Web SEO is easier.

Open Site Explorer is a free tool that gives webmasters the ability to see up to 10,000 links to any website or web page through web Linkscape index.

Linking from some pages of your website to more important pages of your website is always a good idea when it makes sense. Inserting an irrelevant link on a page to another page just for the small SEO benefit should not be done.

If you mean having internal links on your website then yes, it is very important.

If you mean having multiple websites and inter linking them together as your title implies, then definitely not a good idea.

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