Because IPv6 addresses are not supported in the security settings, you will be unable to use =$(CONDOR_HOST)= or =$(COLLECTOR_HOST)= in your security configuration if you specify an IP address.
When using NO_DNS, IPv6 addresses are turned into hostnames by taking the IPv6 address, changing colons to dashes, and appending DEFAULT_DOMAIN_NAME. So 2607:f388:1086:0:21b:24ff:fedf:b520 becomes 2607-f388-1086-0-21b-24ff-fedf-b520.example.com (assuming DEFAULT_DOMAIN_NAME=example.com)
+
+{section: Debugging}
+
+If your HTCondor daemons are having problems communicating, and your logs contain messages like this:
+
+{code}
+> 12/06/12 09:10:59 attempt to connect to <[2607:f388:1086:0:21b:24ff:fedf:b520]:52937> failed: Address family not supported by protocol (connect errno = 97). Will keep trying for 390 total seconds (390 to go).
+{endcode}
+
+The likely cause is that ENABLE_IPV6 has been set in a configuration file specified in LOCAL_CONFIG_FILE or LOCAL_CONFIG_DIR, and not in the global configuration file. The configuration _must_ be in the global configuration file.
