How to create a private Ethereum network
A blockchain is secure because in order to "hack" into the database you need to have 51% of the computational power on the network to create the longest chain. In a large network like Bitcoin or Ethereum the massive computational power of the network ensures generating 51% of the compute power of the network is near impossibly expensive. How secure is a private blockchain? If there are only a few servers the network could be destroyed by just unplugging a few machines, or plugging in a few more.