Hmm, on which host did you tried that? On dev90 I run two processes: java -Djava.net.preferIPv4Stack=true -cp jgroups.jar org.jgroups.tests.McastSenderTest -mcast_addr 224.10.10.10 -bind_addr localhost and java -Djava.net.preferIPv4Stack=true -cp jgroups.jar org.jgroups.tests.McastReceiverTest -mcast_addr 224.10.10.10 -port 5555 -bind_addr localhost And the receiver doesn't see anything. I don't say there is no issues with ipv6 but there still seems to be a change in the behavior with RHEL 4U5 and above. I've opened a bugzilla about that https://bugzilla.redhat.com/show_bug.cgi?id=369591 View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4103972#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...