A computer-implemented method of providing access to a virtual computer network, the method comprising the steps of: providing, by a configurable network service, a programmatic interface for use in configuring virtual computer networks provided by the configurable network service for clients of the configurable network service; receiving, by the configurable network service, from a first client of the configurable network service, configuration information for a first virtual computer network provided by the configurable network service, the configuration information specifying multiple network addresses to be assigned to multiple computing nodes (207a, 207d) of the first virtual computer network, the configuration information further including additional information specified by the first client for use in configuring the first virtual computer network to control communications between the multiple computing nodes of the first virtual computer network, wherein the configuration information is received by the configurable network service via one or more interactions between an executing program, executing on behalf of the first client, and the provided programmatic interface of the configurable network service; automatically configuring the first virtual computer network in accordance with the configuration information, the configuring including associating each of the multiple computing nodes with one of the specified multiple network addresses, the multiple computing nodes being a group of multiple computing nodes of a plurality of computing nodes provided by the configurable network service; automatically configuring the first virtual computer network to allow some communications and to block other communications between the multiple computing nodes in accordance with the specified additional information; and providing, by the configurable network service, access for the first client to the configured first virtual computer network. providing, by a configurable network service, a programmatic interface for use in configuring virtual computer networks provided by the configurable network service for clients of the configurable network service; receiving, by the configurable network service, from a first client of the configurable network service, configuration information for a first virtual computer network provided by the configurable network service, the configuration information specifying multiple network addresses to be assigned to multiple computing nodes (207a, 207d) of the first virtual computer network, the configuration information further including additional information specified by the first client for use in configuring the first virtual computer network to control communications between the multiple computing nodes of the first virtual computer network, wherein the configuration information is received by the configurable network service via one or more interactions between an executing program, executing on behalf of the first client, and the provided programmatic interface of the configurable network service; automatically configuring the first virtual computer network in accordance with the configuration information, the configuring including associating each of the multiple computing nodes with one of the specified multiple network addresses, the multiple computing nodes being a group of multiple computing nodes of a plurality of computing nodes provided by the configurable network service; automatically configuring the first virtual computer network to allow some communications and to block other communications between the multiple computing nodes in accordance with the specified additional information; and providing, by the configurable network service, access for the first client to the configured first virtual computer network.