Cassandra multidatacenter configuration with 1 external ip
You can try to make each node unique for some of the ports, but this isn't really a good path to go down. Cassandra nodes need to see each other on the ports listed here for the best setup: http://www.datastax.com/documentation/cassandra/1.2/index.html#cassandra/security/secureFireWall_r.html

One option that I have seen work well is a host VPN like Openswan: https://www.openswan.org/projects/openswan/