1. A network control system for managing a network comprising a plurality of physical forwarding elements, the system comprising: a first logical forwarding element for logically forwarding data for a first tenant, the first logical forwarding element configurable by the first tenant but not by a second tenant; a second logical forwarding element for logically forwarding data for the second tenant, the second logical forwarding element configurable by the second tenant but not by the first tenant; and a network hypervisor for specifying configuration data for configuring the physical forwarding elements such that the physical forwarding elements implement the first and second logical forwarding elements, wherein the configuration data is subsequently sent to the physical forwarding elements to configure the physical forwarding elements.