Ubuntu 18.04 has switched to netplan for configuring the network interfaces. Netplan generates configurations for NetworkManager or systemd-networkd and effectively replaces ifupdown and the /etc/network/interfaces file.
In an install of Ubuntu Desktop, the default netplan configuration comes from /etc/netplan/01-network-manager-all.yaml which reads:
network: version: 2 renderer: NetworkManager
This basically hands over all network control to NetworkManager. For a static setup we can change the configuration to:
network:
version: 2
renderer: networkd
ethernets:
enp0s25:
dhcp4: no
addresses: [aaa.aaa.aaa.aaa/24]
gateway4: bbb.bbb.bbb.bbb
nameservers:
search: [example.co.uk]
addresses: [xxx.xxx.xxx.xxx,yyy.yyy.yyy.yyy,zzz.zzz.zzz.zzz]
Where enp0s25 is my network interface in this case, the address has a netmask of 255.255.255.0 and the search is the default dns search domain(s) (note this can be vital for getting automounting to work if your setup just uses machine names and assumes the domain is the same).
Note that if you have a laptop you could put this in a file called, say 02_ethernet_interface.yaml and it should override the first configuration for that interface only. I think. Later configurations override earlier ones.
To test:
netplan try
This applies the configuration and then rolls it back in 120 seconds (by default). Press Enter to accept the new settings.
netplan apply
To apply the changes.
For a desktop I just deleted the first config file.
In theory you could probably use this to generate a configuration for NetworkManager (note that on a server you need to explicitly configure NetworkManager to bring up the interface on boot).
