Make a backup copy of the default network interfaces file
cp /etc/network/interfaces /etc/network/default-interfaces
Edit the interfaces file
nano /etc/network/interfaces
Look for these lines, substituting enp0s1 for your own interface
allow-hotplug enp0s1
iface enp0s1 inet dhcp
Edit to this, substituting your own address, gateway, and dns-nameservers.
auto enp0s1
iface enp0s1 inet static
address 192.168.1.100
netmask 255.255.255.0
gateway 192.168.1.1
dns-nameservers 9.9.9.11 149.112.112.11
Save and close the file, restart the network interface
systemctl restart ifup@enp0s1
And restart the networking service
systemctl restart networking.service
Note: If you're using a different service to control networking, restart it instead --
e.g.systemctl restart NetworkManager.service
Now edit (or create) resolv config file
nano /etc/resolv.conf
Add your DNS resolvers into it
nameserver 9.9.9.11
nameserver 149.112.112.11
Confirm your IP is now static with this command
ip -c addr show enp0s1
The output should look something like this
2: enp0s1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
inet 192.168.1.100/24 brd 192.168.0.255 scope global enp0s1