Index: /trunk/src/org/openstreetmap/josm/data/Preferences.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/data/Preferences.java	(revision 5891)
+++ /trunk/src/org/openstreetmap/josm/data/Preferences.java	(revision 5892)
@@ -1328,4 +1328,8 @@
      */
     public void updateSystemProperties() {
+        if(getBoolean("prefer.ipv6", true)) {
+            // never set this to false, only true!
+            updateSystemProperty("java.net.preferIPv6Addresses", "true");
+        }
         updateSystemProperty("http.agent", Version.getInstance().getAgentString());
         updateSystemProperty("user.language", get("language"));