1. A network system comprising: a plurality of host machines, each comprising a non-transitory machine readable medium, for hosting virtual machines, the host machines divided into a plurality of different domains; a plurality of local domain management servers for the plurality of domains, wherein each local domain management server of each domain is for (i) initiating creation of at least one set of distributed virtual switch ports associated with at least one particular logical network identifier on a set of host machines within its domain and (ii) attaching a set of virtual machines on the set of host machines to a created port associated with the particular logical network identifier in order for the virtual machines to send traffic through their respective logical networks; and a second level management server for (i) receiving a specification for a logical network, the specification comprising a plurality of virtual machines to connect to the logical network and a set of at least first and second domains in which the virtual machines are located and (ii) coordinating the use of logical network identifiers between multiple different local domain management servers by selecting a logical network identifier for the logical network and providing the selected logical network identifier to at least first and second local domain management servers with data indicating on which host machines in the respective domains to initiate creation of distributed virtual switch ports in order to allow the logical network to span across at least the first and second local domains so that a first virtual machine in the first local domain communicates via the logical network with a second virtual machine in the second domain.