1. A non-transitory computer-readable medium having stored contents that cause a computing system of an online network service to: obtain information for a virtual computer network having multiple computing nodes provided by the online network service, wherein the information indicates to include one or more external network nodes that are not provided by the online network service and are at an external location that is separate from and outside of a substrate network of the online network service on which the virtual computer network is overlaid; associate, by the computing system of the online network service, one or more virtual network addresses for the virtual computer network with the one or more external network nodes to include the one or more external network nodes within the virtual computer network; and manage, by the computing system of the online network service, a communication sent from one of the multiple computing nodes to one of the associated one or more virtual network addresses by: determining that a module of the online network service in the substrate network is configured to manage communications for the one or more external network nodes; and forwarding the communication over the substrate network to the module, to enable the module to further forward the communication outside the substrate network to at least one of the one or more external network nodes.