Time Capsules and Mac Server-based network drives for Time Machine targets. So basically you either have to buy a time capsule or attach a plain old external USB hard drive to the Mac to use it with Time Machine. Or if you really want it to work over the network, you have to setup a Mac Server or you have set it up with Linux and spoof Mac server using AFP and Netatalk. The latter doesn't work very well. I set it up, but every few weeks I would get an error and Time Machine would redo the initial backup which takes hours. And I wouldn't dare trust it to actually be functioning the rest of the time either.