Quantum is a virtual network service for Openstack, and a part of Netstack.

Just like OpenStack Nova provides an API to dynamically request and configure virtual servers, Quantum provides an API to dynamically request and configure virtual networks. These networks connect "interfaces" from other OpenStack services (e.g., vNICs from Nova VMs). The Quantum API supports extensions to provide advanced network capabilities (e.g., QoS, ACLs, network monitoring, etc)

Quantum is an officially incubated OpenStack project.