A network control system for managing a set of forwarding elements (1220-1230; 1240-1250) in a network, the network control system comprising: a first set of network controllers arranged to configure a first plurality of forwarding elements that enable communication between a first plurality of machines (VM1, VM2) by forwarding traffic between the first plurality of machines (VM1, VM2); a second set of network controllers arranged to configure a second plurality of forwarding elements that enable communication between a second plurality of machines (VM3, VM4) by forwarding traffic between the second plurality of machines (VM3, VM4), wherein the second plurality of forwarding elements are separate from the first plurality of forwarding elements, and the second plurality of machines (VM3, VM4) are separate from the first plurality of machines (VM1, VM2); and characterized in that: the first set of network controllers are arranged to define a first logical datapath set to which the machines of the first plurality (VM1, VM2) logically connect; the second set of network controllers are arranged to define a second logical datapath set to which the machines of the second plurality (VM3, VM4) logically connect; the network control system further comprises a third set of network controllers for managing the first and second sets of network controllers (1215; 1235) in order to enable communication between machines in the first plurality of machines (VM1, VM2) and machines in the second plurality of machines (VM3, VM4), wherein the first and second pluralities of forwarding elements (1220-1230; 1240-1250) forward traffic between machines in the first plurality (VM1, VM2) and machines in the second plurality (VM3, VM4), and wherein the third set of network controllers are arranged to define a third logical datapath set to which the machines of both the first and second pluralities of machines (VM1, VM2; VM3, VM4) logically connect. a first set of network controllers arranged to configure a first plurality of forwarding elements that enable communication between a first plurality of machines (VM1, VM2) by forwarding traffic between the first plurality of machines (VM1, VM2); a second set of network controllers arranged to configure a second plurality of forwarding elements that enable communication between a second plurality of machines (VM3, VM4) by forwarding traffic between the second plurality of machines (VM3, VM4), wherein the second plurality of forwarding elements are separate from the first plurality of forwarding elements, and the second plurality of machines (VM3, VM4) are separate from the first plurality of machines (VM1, VM2); and characterized in that: the first set of network controllers are arranged to define a first logical datapath set to which the machines of the first plurality (VM1, VM2) logically connect; the second set of network controllers are arranged to define a second logical datapath set to which the machines of the second plurality (VM3, VM4) logically connect; the network control system further comprises a third set of network controllers for managing the first and second sets of network controllers (1215; 1235) in order to enable communication between machines in the first plurality of machines (VM1, VM2) and machines in the second plurality of machines (VM3, VM4), wherein the first and second pluralities of forwarding elements (1220-1230; 1240-1250) forward traffic between machines in the first plurality (VM1, VM2) and machines in the second plurality (VM3, VM4), and wherein the third set of network controllers are arranged to define a third logical datapath set to which the machines of both the first and second pluralities of machines (VM1, VM2; VM3, VM4) logically connect. the first set of network controllers are arranged to define a first logical datapath set to which the machines of the first plurality (VM1, VM2) logically connect; the second set of network controllers are arranged to define a second logical datapath set to which the machines of the second plurality (VM3, VM4) logically connect; the network control system further comprises a third set of network controllers for managing the first and second sets of network controllers (1215; 1235) in order to enable communication between machines in the first plurality of machines (VM1, VM2) and machines in the second plurality of machines (VM3, VM4), wherein the first and second pluralities of forwarding elements (1220-1230; 1240-1250) forward traffic between machines in the first plurality (VM1, VM2) and machines in the second plurality (VM3, VM4), and wherein the third set of network controllers are arranged to define a third logical datapath set to which the machines of both the first and second pluralities of machines (VM1, VM2; VM3, VM4) logically connect.