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DNS is one of the simplest yet most misunderstood and undervalued parts of the entire internet system.
If DNS is not setup correctly your website(s) will not display, or if your ISP’s DNS is having issues you won’t be able to visit websites either.
Of course there are also nasty things that can happen if DNS is exploited.
If you want to be sure that your provider’s dns is setup correctly then you can use a handy little tool provided by IANA.
You can read more about the tool and what it does on the ICANN site