1. A computer-implemented method for managing a network, the method comprising: receiving, by one or more computing systems, communications sent between a plurality of computing nodes of a virtual computer network that is overlaid on a substrate network interconnecting the plurality of computing nodes, each of the communications being directed to one of multiple virtual Internet Protocol (IP) addresses for the virtual computer network, each of the plurality of computing nodes having an associated substrate network address for the substrate network; forwarding, by the one or more computing systems and for each of the received communications, the received communication to a destination one of the plurality of computing nodes that is identified by the one virtual IP address to which the received communication is directed, the forwarding using the associated substrate network address for the destination one computing node; assigning, by a system manager computing system configured to manage operations of the virtual computer network, a first of the multiple virtual IP addresses to a pool of multiple of the plurality of computing nodes that are alternative destinations for communications sent to the first virtual IP address; selecting, by the system manager computing system and for a first of the plurality of computing nodes, a second computing node to associate with the first virtual IP address for future communications sent by the first computing node to the first virtual IP address, the second computing node being one of the multiple computing nodes of the pool and being associated with a second substrate network address; and after the selecting of the second computing node to associate with the first virtual IP address for future communications sent by the first computing node to the first virtual IP address, receiving, by the one or more configured computing systems, one or more communications sent by the first computing node to the first IP address; and forwarding each of the one or more communications to the second computing node using the second substrate network address.