Fix subnet config for DHCPv6 server

Change-Id: Ica5ef5d16ea022ea96ec8c6a1e005c0506798c2d
diff --git a/trellis/trellis_dualhome.json b/trellis/trellis_dualhome.json
index 7fd6d6b..4fd2a89 100644
--- a/trellis/trellis_dualhome.json
+++ b/trellis/trellis_dualhome.json
@@ -99,7 +99,7 @@
         "of:0000000000000205/10" : {
             "interfaces" : [
                 {
-                    "ips" : [ "10.0.3.254/24" ],
+                    "ips" : [ "10.0.3.254/24", "2000::3ff/120" ],
                     "vlan-untagged": 30
                 }
             ]