JGroups needs to support both IPv4 and IPv6. When running JGroups on a machine which has a dual IP stack, two system properties need to be set in order to specify preferences for one version of IP over the other:

(excerpt from Sun notes)
1. IPv6 stack is preferred by default, since on a dual-stack machine IPv6 socket can talk to both IPv4 and IPv6 peers.
To change this, set -Djava.net.preferIPv4Stack=true
(NOTE: if we use java.net.preferIPv4Stack=true and use IPv6 addresses, we get a message "exception while trying to create socket:java.net.SocketException: Protocol family unavailable")

2. By default, we would prefer IPv4 addresses over IPv6 addresses (i.e., when querying the name service (e.g., DNS service), we would return IPv4 addresses ahead of IPv6 addresses).
There are two reasons for this choice:

1. There are some applications that expect an IPv4 address textual format, i.e. "%d.%d.%d.%d". Using an IPv4 address minimizes the surprises;
2. Using IPv4 address, we can use one call (with an IPv6 socket) to reach either a legacy IPv4-only service, or an IPv6 service (unless the IPv6 service is on a Ipv6 only node).

To change this, set -Djava.net.preferIPv6Addresses=true

This issue concerns modifications to the JGroups testsuite to allow easy switching from IPv4 to IPv6 for testing purposes.