Officially, you can use Time Machine to back up to a connected hard drive, or to a Time Capsule or Airport Extreme hard drive. But an unsupported mode allows you to back up to a drive connected to another computer on your network. You’ll need to run the Terminal program and execute the line “defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1”. Then mount the network drive you wish to use. Then you should see it turn up in the drives you can select in the Time Machine preferences.