1. A method implemented by data processing apparatus, the method comprising: maintaining, by the data processing apparatus, data that associates, for each of a plurality of virtual machines in a first cluster of virtual machines launched by a first user, a unique name for the virtual machine with an internal network address for the virtual machine, wherein the internal network address for the virtual machine is a network address used by other virtual machines in the first cluster to communicate with the virtual machine over an internal network; receiving, by a process executing on the data processing apparatus and from a source virtual machine executing on the data processing apparatus, a first outgoing packet destined for a recipient; determining that the first outgoing packet identifies the recipient by a unique name for the recipient instead of by a network address for the recipient; in response to determining that the first outgoing packet identifies the recipient by the unique name for the recipient instead of by a network address for the recipient, determining, by the process executing on the data processing apparatus and from the maintained data, that the unique name is associated in the maintained data with an internal network address of a destination virtual machine that belongs to the first cluster of virtual machines, wherein the unique name for the destination virtual machine is assigned to the destination virtual machine by a name association system when the destination virtual machine is launched; and sending the first outgoing packet to the internal network address of the destination virtual machine.