Mac OS normally doesn’t allow you to use network shares as targets for TimeMachine backups. This can be worked around, tho. 
First you need to tweak Mac OS to accept network shares by entering the following command in a Terminal session:
defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1
That would already allow you to store your backups on a network share. But do you really want to trust your valuable data to a network share that can potentially be accessed by untrusted users, such as your favorite bastard admin from hell?!
Here’s when the following comes in:
You will create an encrypted sparse bundle and use it as a target for the backup.
