How can I use a "Backup server"?

[narutodude000]

I'm using a shared server. I want to use a free backup server to deliver a "Server down" message if my primary (paid) server goes down. How can I do this?

[narutodude000]

I did some research, is this similar to "CDN"? How exactly do I set this up?
I have shared hosting, my host (Justhost) has many features that other hosts do not have. Do you guys think it's possible that they have the feature to set up CDN or a backup server?