If you are running an apache server local and you use your local IP (127.0.0.1), then you will get an IP address.
ServerName gives the name and port that the server uses to identify itself.
This can often be determined automatically, but we recommend you specify
it explicitly to prevent problems during startup.
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If this is not set to valid DNS name for your host, server-generated
redirections will not work. See also the UseCanonicalName directive.
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If your host doesn't have a registered DNS name, enter its IP address here.
You will have to access it by its address anyway, and this will make
redirections work in a sensible way.
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ServerName 127.0.0.1:80