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What advantage does different domains have.
For example what is the advantage of using .com instead of .tk
 And when buying domains, do you buy it once or you keep renewing it?
Thanks

There's no advantage other than making the user type more or less letters when trying to go to your forum. A .com domain looks more professional and is most common. The domain often represents what your website is about, for an example, .org stands for organization so organizations can use that domain to let the user know that they are an organization. You're charged money per year to keep a domain so keep renewing if you still want it.