Notes about Bionic Beaver, Ubuntu LTS 18.01

sudo touch /etc/cloud/cloud-init.disabled
sudo echo "network: {config: disabled}" > /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg

Also update this

FCheck for the existence of /etc/cloud/cloud.cfg configuration. If the file exists edit the file and change the settings within:
FROM:
preserve_hostname: false
TO:
preserve_hostname: true

sudo hostnamectl set-hostname NEWHOSTNAME

In this case a Bridge network on 192.168.1.1/24 and a host-only network on 192.168.56.1/24. Most important is the absence of the gateway4 line in the host-only network to ensure internet traffic goes via 192.168.1.1 gateway.

network:
    version: 2
    renderer: networkd
    ethernets:
        enp0s3:
            dhcp4: no
            addresses: [192.168.1.60/24]
            gateway4: 192.168.1.1
            nameservers:
              addresses: [8.8.8.8,192.168.1.1]
              search: [ example.org ]
        enp0s8:
            dhcp4: no
            addresses: [192.168.56.60/24]

Routing table output

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.1.1     0.0.0.0         UG    0      0        0 enp0s3
192.168.1.0     0.0.0.0         255.255.255.0   U     0      0        0 enp0s3
192.168.56.0    0.0.0.0         255.255.255.0   U     0      0        0 enp0s8