Can you see if iptables is running ? sudo service iptables status . If it is running please run sudo service iptables stop. I am guessing Iptables have a reject rule which is stopping traffic at that port.
The other thing to check is : while doing the installation when it asks for cassandra IP if you specify 127.0.0.1 or localhost it will fail. Instead you should provide there what hostname -i returns .
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