1. A method for a computing system to authorize communications between computing nodes, the method comprising: under control of a first communication manager module that manages communications for multiple associated computing nodes, receiving an incoming communication intended for an executing program on a destination node that is one of the associated computing nodes for the first communication manager module, the incoming communication having an indicated source network address corresponding to a remote sending node that sent the incoming communication and that is one of multiple other computing nodes managed by a remote second communication manager module, the first communication manager module being communicatively connected to the second communication manager module via at least one intermediate physical network; before forwarding the incoming communication to the destination node, automatically determining whether the incoming communication is authorized for the destination node based at least in part on the indicated source network address for the incoming communication, the automatic determining including: extracting from the indicated source network address a partial source network address that corresponds to the second communication manager module, a virtual network address for the sending node that is based on a virtual local network to which the sending node and destination node belong, and a network identifier that uniquely identifies the virtual local network; interacting with a remote system manager module that maintains information about computing nodes associated with multiple communication manager modules, the interacting including providing an indication of the extracted virtual network address and extracted network identifier and including receiving an indication of a partial network address corresponding to an indicated communication manager module whose associated computing nodes include the computing node to which is assigned the extracted virtual network address for the virtual network having the extracted network identifier; verifying whether the partial network address indicated by the system manager module matches the partial source network address extracted from the indicated source network address for the incoming communication, and whether the sending node is authorized to send the incoming communication to the destination node based at least in part on predetermined authorization rules; and determining that the incoming communication is authorized for the destination node if the partial network address indicated by the system manager module is verified to match the partial source network address extracted from the indicated source network address for the incoming communication and if the sending node is verified to be authorized to send the incoming communication to the destination node; and if the incoming communication is automatically determined to be authorized for the destination node, initiating forwarding of the incoming communication to the destination node.