Open Parent Directory 
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 DEBIAN - APACHE2 - SET THE SERVERNAME DIRECTIVE GLOBALLY
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 If you get an ERROR for not setting the globally 'ServerName'-Variable
 like in this example:
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AH00558: apache2: Could not reliably determine the server's fully qualified
domain name, using fe80::1:dcff:fe2f:d311%ens6. Set the 'ServerName' directive
globally to suppress this message.

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 Then try as follow:
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Show all ConfigurationFiles, including of 'ServerName' Variable:

$ sudo find /etc/apache2 -type f -exec grep -s ServerName {} \;

Take a line at the Begin of the ConfigurationFile, like: 'ServerName localhost' or fixed IPV4 instead
to set the directive globally:

$ sudo nano /etc/apache2/sites-enabled/000-default.conf

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 Description/Facts:
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IPv6 addresses that start with fe80 are so-called link-local addresses,
they are only valid within a same network-switch and are never routed.