apache2: Could not reliably determine the server's fully qualified domain name

When I do:

$ sudo service apache2 restart

it will print the following messages:

apache2: Could not reliably determine the server's fully qualified domain name, using 127.0.0.1 for ServerName
 ... waiting .apache2: Could not reliably determine the server's fully qualified domain name, using 127.0.0.1 for ServerName
                                                                                                                                                                  [ OK ]

How to fix the problem?

$ sudo vi /etc/apache2/apache2.conf

Add the following line to this configuration file:

ServerName yourservername

And then try again:

$ sudo service apache2 restart
 * Restarting web server apache2                                                                                                                                          ... waiting                                                                                                                                                      [ OK ]