1. A network controller instance for managing a network comprising a plurality of physical forwarding elements, the network controller instance comprising: a set of processing units; and a machine readable medium storing, for execution by the set of processing units: a control application for enabling a plurality of tenants to define logical forwarding elements and associated logical gateways, wherein each particular logical forwarding element specifies logical connections between virtual machines (VMs) in a set of VMs and the associated logical gateway for the particular logical forwarding element specifies a connection between the set of VMs and other entities that do not connect to the logical forwarding element, wherein different tenants specify different switching configurations for the logical forwarding elements while remaining insulated from a topology of the network; a network hypervisor for generating provisioning data for provisioning sets of the physical forwarding elements to implement the logical forwarding elements; and a network operating system for distributing the provisioning data to the sets of physical forwarding elements in order for the physical forwarding elements in the sets to implement the logical forwarding elements.