Abstract: A data center can share processing resources using virtual networks. A virtual machine manager (10) hosts one or more virtual machines (11, 411), the virtual machines forming part of a segmented virtual network (34). Outgoing messages from the virtual machines have an intermediate destination address of an intermediate node in a local segment of the segmented virtual network, and the virtual machine manager has a router (18) for determining a new intermediate destination address outside the local segment, for routing the given outgoing message. By having the router as part of the virtual machine manager rather than having only a switch in the virtual machine manager, the need for virtual machines for implementing gateways is avoided. This can reduce the number of “hops” for the message between virtual entities hosted, and thus improve performance. This can help a service provider to share physical processing resources of a data center between different clients having their own virtual networks.