Comment 2 for bug 1642303

Also, so we're sure we're actually following a similar use case, what I did to actually test was to update tools/init-runonce for my external network:

# git diff tools/init-runonce
diff --git a/tools/init-runonce b/tools/init-runonce
index 7fb50f7..248cdc2 100755
--- a/tools/init-runonce
+++ b/tools/init-runonce
@@ -56,7 +56,8 @@ if [[ "${NETWORK_MANAGER}" == "nova" ]] ; then
 else
     echo Configuring neutron.
     neutron net-create public1 --router:external --provider:physical_network physnet1 --provider:network_type flat
- neutron subnet-create --name 1-subnet --disable-dhcp --allocation-pool start=10.0.2.150,end=10.0.2.199 public1 10.0.2.0/24 --gateway 10.0.2.1
+ neutron subnet-create --name 1-subnet --disable-dhcp --allocation-pool start=192.168.6.100,end=192.168.6.150 public1 192.168.6.0/24 --gateway 192.168.6.1
+
     neutron net-create demo-net --provider:network_type vxlan
     neutron subnet-create demo-net 10.0.0.0/24 --name demo-subnet --gateway 10.0.0.1 --dns-nameservers list=true 8.8.8.8
     neutron router-create demo-router

Then:

tools/init-runonce
openstack server create --image cirros --flavor m1.tiny --key-name mykey --nic net-id=cb8ae6de-bf97-4a23-a009-bf6b7d173e0b demo1
openstack floating ip create public1
openstack server add floating ip demo1 192.168.6.109
# ping 192.168.6.109
PING 192.168.6.109 (192.168.6.109) 56(84) bytes of data.
64 bytes from 192.168.6.109: icmp_seq=1 ttl=63 time=7.23 ms
64 bytes from 192.168.6.109: icmp_seq=2 ttl=63 time=12.9 ms
^C