1. A non-transitory computer-readable storage medium comprising stored contents that configure a computing system to: overlay a virtual computer network on a substrate network in accordance with information indicating that a first computing node of the virtual computer network is configured to act as an intermediate destination for at least some communications sent via the virtual computer network, wherein the information is configuration information specified by a user on whose behalf the virtual computer network is provided and includes a specified network topology for the virtual computer network, the specified network topology including the first computing node being located between a first group of one or more computing nodes in the virtual computer network and a second group of one or more other computing nodes; and manage a communication for the virtual computer network that is sent by a source computing node in the virtual computer network to an indicated destination network address associated with an intended final destination of the communication, the managing of the communication including: determining to use the first computing node as an intermediate destination for the communication; and causing the communication to be sent over the substrate network to the first computing node before the communication is further sent to the destination network address.