SSH protocol is used to exchange data between two networked devices using a secure channel. Ubuntu server machines with ssh installed on them can be reached from other computers on the network. Ubuntu's synaptic package manager easily installs ssh for your server machine. In order to reach remote ubuntu server machines with ssh installed on them, type the following command from command line of the client machine:
$ ssh remoteUserName@remoteMachineIPAddress
replace remoteUserName and remoteMachineIPAddress with appropriate values. Then you will see that it is asking for adding to the list of known hosts. write 'yes' for confirmation. Then type following from the console window and press enter, the remote machine will shutdown.
$ sudo shutdown -h now