Ports 57311 and 57312 are randomly assigned ports used for RMI communication. These ports change each time Cassandra starts up, but need to be open in the firewall, along with 8080/7199 (depending on version), to allow for remote JMX access. Something that doesn't appear to be particularly well documented, but has tripped me up in the past.
1024 - 65355 Random port required by JMX. Starting with Java 7u4 a specific port can be specified using the com.sun.management.jmxremote.rmi.port property.
7000 Inter-node cluster
7001 SSL inter-node cluster
9042 CQL Native Transport Port
9160 Thrift
DataStax OpsCenter
61620 opscenterd daemon
61621 Agent
8888 Website
Architecture
A possible architecture with Cassandra + OpsCenter on EC2 could look like this: