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What is the difference between using www. before domain name or ignoring? Why some domain names gets resolved fine either with www. prefix or without, while others fail?

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up vote 6 down vote accepted

www is technically the subdomain of that website. It's usually the default and primary domain; however, sometimes a domain provider will set their websites up such that http://www.domain.com and http://domain.com point to separate places.

In other words, it's convention.

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It depends on how the person being responsible for the domain has defined his/her setup. When you register a domain, it is registered as "domain.tld" only. The "www" in front of "domain.tld" is only a very common subdomain, but not really part of the "original domain".

When you want to use www.domain.tld, you either have to add the according DNS entry or configure an URL rewrite.

In most cases, a website is shown when typing in "www" or not.

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