diff --git a/utils/he_ipv6/tunnel.py b/utils/he_ipv6/tunnel.py
index cd134c2..3f47ad4 100644
--- a/utils/he_ipv6/tunnel.py
+++ b/utils/he_ipv6/tunnel.py
@@ -1,5 +1,6 @@
 import ipaddress
 import logging
+import re
 import socket
 ##
 import netaddr
@@ -130,11 +131,16 @@ class Assignment(object):
         self.iface_blocks = self.alloc_block.extract_subnet(self.prefix, count = 1)
         logger.debug('Allocation blocks for {0}: {1}'.format(self.iface, ','.join([str(i) for i in self.iface_blocks])))
         for idx, i in enumerate(self.iface_blocks):
+            if i.prefixlen > 64:
+                raise ValueError('Allocation block must be a /64 or larger')
             # DHCPv6 range.
-            _base = '{0}:{1}'.format(str(i.ip).rstrip(':'), idx)
+            # We need to do some funky things here. Netaddr doesn't have an .exploded().
+            _base = ipaddress.IPv6Address(str(i.ip))
+            _base = ipaddress.IPv6Address(re.sub(r'(:0000){4}$', r':dead:beef:cafe::', str(_base.exploded)))
+            _base = re.sub(r':0$', r'', _base)
             logger.debug('Base prefix for {0}: {1}'.format(str(i), _base))
-            start = '{0}:dead:beef:cafe:0'.format(_base)
-            stop = '{0}:dead:beef:cafe:ffff'.format(_base)
+            start = '{0}:0'.format(_base)
+            stop = '{0}:ffff'.format(_base)
             d_range = (start, stop)
             self.dhcp6_ranges.append(d_range)
             logger.debug('Added range {0} to block {1} for iface {2}'.format(d_range, str(i.ip), self.iface))