# virsh net-list --all
Name                 State      Autostart
-----------------------------------------
default              active     yes


#cd /usr/share/libvirt/networks

# vim heartbeat.xml

<network>
  <name>heartbeat</name>
  <bridge name="virbr1"/>
  <forward/>
  <ip address="10.10.10.1" netmask="255.255.255.0">
    <dhcp>
      <range start="10.10.10.10" end="10.10.10.254"/>
    </dhcp>
  </ip>
</network>


# virsh net-define /usr/share/libvirt/networks/heartbeat.xml

Mark the default network to automatically start:
# virsh net-autostart heartbeat

Start the default network:
# virsh net-start default