external IP address as FQDN for VM GPL?

I have a fresh and updated install of Ubuntu 18.04.1 and want to set up VM GPL for testing and eventual use. I know the domain name I’ll eventually use but not for several months yet. I think I could use the eventual domain name as FQDN but can see lotsa issues with mail and updates etc. Or I’ve thought about just using xxx.xxx.xxx.xxx since I have a static external IP. Anyone have experience or advice on this?