I have installed open stack using devstack. Launched an cirros image. I can connect to the instance from VNC console but unable to ping to the instance.
You need to assign a floating IP to that instance. After that you'll be able to access the instance by that floating IP.
If you want to access any extra services on your instances you will also need to configure a proper security group and assign your instances to it.
NOTE: If you are running devstack inside a VM that is behind NAT, you won't be able to ping instances from outside that devstack VM without proper networking hacks.