The Book

https://www.techrepublic.com/article/how-to-configure-a-static-ip-address-in-ubuntu-server-18-04/

Gone are the days of manually editing the flat text <fc #ff0000>/etc/network/interfaces</fc> file. In its place is netplan. That's right, Ubuntu fans, the method you've known for years is now a thing of the past. Instead of a very simple text file, Ubuntu Server requires editing a .yaml file (complete with proper adherence to correct code indent for each line of the block), in order to configure your IP addressing.