Hi ,

Check /etc/resolv.conf file and change your name-server to 8.8.8.8

Can you check the default GW. Also, you should be checking if DNS is added in your resolv.conf

I am assuming you are using Neutron ML@ OpenvSwitch with VXLAN or GRE.

1.) After you create the instance, create a Router.
2.) Router should have two interfaces, one connected to the
tenant private network. and second
to external network (Default
gateway)
3.) From instance ping the router.
4.) From the instance ping external IP to OpenStack. i.e physical
gateway outside the Public Network.
5.) you must also check your security group permissions.
6.) Try to assign floating IP to instance and try ratability from
external network to instance.
7.) Please note that External network connectivity is via Network
Node, and not directly from the Nova
(Unless you are using Provider
networks.

Hope that Helps.
-VJ.