The DO tutorial is a good start, but as another poster mentioned further down, check out: https://github.com/konstruktoid/hardening

note: the DO tutorial will hold your hand a little; the hardening doc expects a (minor) degree of familiarity

Thanks. I saw this git repo earlier and it looks interesting (even though most of my machines are on LTS 18).

I don't see anything about subnets in there though. Did I miss something?