The system hostname is configured in /etc/hostname. A change made to this file will take effect after reboot.
At boot time the string in /etc/hostname is loaded into the kernel using the
hostname command. If you edit /etc/hostname then you should also set the system hostname manually using the
sudo hostname mismis
Note that in Debian and Ubuntu the system hostname is the short hostname, not a fully qualified domain name.
If the machine has a static external IP address AAAA then this address and the hostname should appear on a line in /etc/hosts, as follows.
If the machine has a static fully-qualified domain name then this should appear first after the IP address on the line and the short hostname should follow it.
AAAA <fully-qualified-hostname> <short-hostname>
In the present case, assuming that the external IP address is 188.8.131.52 and the desired canonical hostname is "mismis.com", the /etc/hosts line should look like the following.
184.108.40.206 mismis.com mismis
If the machine has no static external IP address, but gets assigned addresses dynamically via DHCP, for example, then 127.0.1.1 should be used instead.
127.0.1.1 mismis.com mismis
In /etc/hosts, any IP address should appear on at most ONE line. On that line the first domain name that follows the IP address is the canonical hostname associated with that IP address, and any subsequent domain names are aliases for the canonical hostname.