This article describes a couple of caveats with Rackspace Cloud.
When you have created the Cloud Servers on Rackspace then the public IPs will be the default IP.
If you do hostname -i as follows you will get the public IP (you can use the public IP too, but Rackspace will charge you more, and from a security stand point you may also want to stick to private IPs):
But wait a minute, using the Public IP for Cluster Communication will cost more that using the internal IP! To fix this you have to determine the Internal IP of the Cloud Server. It can be done using Rackspace APIs or as follows:
root@monitor:~#ifconfig |grep "inet addr"
inet addr:184.108.40.206 Bcast:220.127.116.11 Mask:255.255.255.0
inet addr:10.177.4.180 Bcast:10.177.63.255 Mask:255.255.192.0
inet addr:127.0.0.1 Mask:255.0.0.0
The internal IP is highlighted in bold.
Collect the private IP from all your Rackspace instances and use the Private IPs in the Configurator of your choice.