diff -Nru isc-dhcp-4.2.4/debian/changelog isc-dhcp-4.2.4/debian/changelog
--- isc-dhcp-4.2.4/debian/changelog	2016-04-15 08:44:06.000000000 -0400
+++ isc-dhcp-4.2.4/debian/changelog	2016-08-04 13:26:58.000000000 -0400
@@ -1,3 +1,10 @@
+isc-dhcp (4.2.4-7ubuntu12.6) trusty; urgency=medium
+
+  * Don't assume IPv6 prefix length of 64 (LP: #1609898).
+    Pulled from debian commit c347ab8a43587164486ce1f104eedfd638594e59.
+
+ -- Dan Streetman <dan.streetman@canonical.com>  Thu, 04 Aug 2016 13:07:23 -0400
+
 isc-dhcp (4.2.4-7ubuntu12.5) trusty; urgency=medium
 
   * debian/apparmor-profile.dhclient: Add the attach_disconnected flag to prevent
diff -Nru isc-dhcp-4.2.4/debian/dhclient-script.linux isc-dhcp-4.2.4/debian/dhclient-script.linux
--- isc-dhcp-4.2.4/debian/dhclient-script.linux	2014-04-03 17:51:15.000000000 -0400
+++ isc-dhcp-4.2.4/debian/dhclient-script.linux	2016-08-04 13:26:58.000000000 -0400
@@ -400,9 +400,9 @@
         ;;
 
     BOUND6|RENEW6|REBIND6)
-        if [ "${new_ip6_address}" ] && [ "${new_ip6_prefixlen}" ]; then
+        if [ "${new_ip6_address}" ]; then
             # set leased IP
-            ip -6 addr add ${new_ip6_address}/${new_ip6_prefixlen} \
+            ip -6 addr add ${new_ip6_address} \
                 dev ${interface} scope global
         fi
 
@@ -416,23 +416,19 @@
         ;;
 
     DEPREF6)
-        if [ -z "${cur_ip6_prefixlen}" ]; then
-            exit_with_hooks 2
-        fi
-
         # set preferred lifetime of leased IP to 0
-        ip -6 addr change ${cur_ip6_address}/${cur_ip6_prefixlen} \
+        ip -6 addr change ${cur_ip6_address} \
             dev ${interface} scope global preferred_lft 0
 
         ;;
 
     EXPIRE6|RELEASE6|STOP6)
-        if [ -z "${old_ip6_address}" ] || [ -z "${old_ip6_prefixlen}" ]; then
+        if [ -z "${old_ip6_address}" ]; then
             exit_with_hooks 2
         fi
 
         # delete leased IP
-        ip -6 addr del ${old_ip6_address}/${old_ip6_prefixlen} \
+        ip -6 addr del ${old_ip6_address} \
             dev ${interface}
 
         ;;