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DNS is the acronym for "Domain name service". It is a service that translates site names into their actual address. For example, community.atlassian.com is not a site address, it is a name that has several real IP addresses behind it. You use DNS to lookup a real address given a name.
You automatically "check the DNS" every time you specify a remote host name instead of an actual IP address to connect to with a browser, a REST call, a remote shell (telnet or ssh for example), and anything else you might do over a network. The whole point of using DNS is that you resolve a domain to the actual address it is present on.
So, I am not sure what you mean by "when should I check the DNS". What are you trying to do here?
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