Index: trunk/src/org/openstreetmap/josm/gui/MainApplication.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/MainApplication.java	(revision 8705)
+++ trunk/src/org/openstreetmap/josm/gui/MainApplication.java	(revision 8706)
@@ -16,5 +16,7 @@
 import java.net.Inet6Address;
 import java.net.InetAddress;
+import java.net.InetSocketAddress;
 import java.net.ProxySelector;
+import java.net.Socket;
 import java.net.URL;
 import java.security.AllPermission;
@@ -558,4 +560,8 @@
                             if (a instanceof Inet6Address) {
                                 if (a.isReachable(1000)) {
+                                    /* be sure it REALLY works */
+                                    Socket s = new Socket();
+                                    s.connect(new InetSocketAddress(a, 80), 1000);
+                                    s.close();
                                     Utils.updateSystemProperty("java.net.preferIPv6Addresses", "true");
                                     if (!wasv6) {