systemctl start networking.service

networking.service raises or downs the network interfaces configured in /etc/network/interfaces, that is, those network interfaces which are not managed by NetworkManager. If you look into /lib/systemd/system/networking.service you will see that all it does is ifup or ifdown depending on whether it is to be started or stopped.

https://askubuntu.com/questions/850339/what-is-the-networking-service