diff --git a/src/etc/inc/plugins.inc.d/dhcpd.inc b/src/etc/inc/plugins.inc.d/dhcpd.inc
index 21587b520..99bc0c07b 100644
--- a/src/etc/inc/plugins.inc.d/dhcpd.inc
+++ b/src/etc/inc/plugins.inc.d/dhcpd.inc
@@ -1318,6 +1318,8 @@ function dhcpd_dhcp6_configure($verbose = false, $blacklist = array())
                 /* with enough room we can add dhcp6 prefix delegation */
                 $pdlen = calculate_ipv6_delegation_length($config['interfaces'][$ifname]['track6-interface']);
                 if ($pdlen > 2) {
+                    /* XXX calculation is probably out of whack, please fix */
+
                     $pdlenmax = $pdlen;
                     $pdlenhalf = $pdlenmax - 1;
                     $pdlenmin = 64 - ceil($pdlenhalf / 4);
@@ -1345,10 +1347,25 @@ function dhcpd_dhcp6_configure($verbose = false, $blacklist = array())
                 }
 
                 if (!empty($dhcpdv6cfg[$ifname]['prefixrange']['from']) && !empty($dhcpdv6cfg[$ifname]['prefixrange']['to'])) {
-                    $pdval = intval($dhcpdv6cfg[$ifname]['prefixrange']['prefixlength']);
+                    /* XXX $pdlen is never validated against prefixlenght setting, but must be smaller or equal */
+                    $pdlen = 64 - calculate_ipv6_delegation_length($config['interfaces'][$ifname]['track6-interface']);
 
-                    $dhcpdv6cfg[$ifname]['prefixrange']['from'] = merge_ipv6_address($ifcfgipv6, $dhcpdv6cfg[$ifname]['prefixrange']['from'], $pdval);
-                    $dhcpdv6cfg[$ifname]['prefixrange']['to'] = merge_ipv6_address($ifcfgipv6, $dhcpdv6cfg[$ifname]['prefixrange']['to'], $pdval);
+                    $range_from = merge_ipv6_address($dhcpdv6cfg[$ifname]['prefixrange']['from'], '::');
+                    if ($range_from == '::') {
+                        log_error("Warning! '{$dhcpdv6cfg[$ifname]['prefixrange']['from']}' is not a valid prefix range value");
+                        /* XXX previously it was suggested to use suffix but it was actually infix so shift 64 bits if possible */
+                        $range_from = $dhcpdv6cfg[$ifname]['prefixrange']['from'] . ':0:0:0:0';
+                    }
+
+                    $range_to = merge_ipv6_address($dhcpdv6cfg[$ifname]['prefixrange']['to'], '::');
+                    if ($range_to == '::') {
+                        log_error("Warning! '{$dhcpdv6cfg[$ifname]['prefixrange']['to']}' is not a valid prefix range value");
+                        /* XXX previously it was suggested to use suffix but it was actually infix so shift 64 bits if possible */
+                        $range_to = $dhcpdv6cfg[$ifname]['prefixrange']['to'] . ':0:0:0:0';
+                    }
+
+                    $dhcpdv6cfg[$ifname]['prefixrange']['from'] = merge_ipv6_address($ifcfgipv6, $range_from, $pdlen);
+                    $dhcpdv6cfg[$ifname]['prefixrange']['to'] = merge_ipv6_address($ifcfgipv6, $range_to, $pdlen);
                 }
             }
         }
diff --git a/src/www/services_dhcpv6.php b/src/www/services_dhcpv6.php
index 7dccefa8e..ec4df7c37 100644
--- a/src/www/services_dhcpv6.php
+++ b/src/www/services_dhcpv6.php
@@ -554,7 +554,7 @@ include("head.inc");
                             'calculator to ensure you have entered a correct range if the dhcpd6 server fails to start.') ?>
 <?php if (isset($config['interfaces'][$if]['dhcpd6track6allowoverride'])): ?>
                           <br/><br/>
-                          <?= gettext('When using a tracked interface then please only enter the range itself. i.e. ::xx. For example, for a /60 subnet from ::20 to ::40.') ?>
+                          <?= gettext('When using a tracked interface then please only enter the range itself. i.e. ::xxxx:0:0:0:0. For example, for a /56 subnet from ::100:0:0:0:0 to ::f00:0:0:0:0.') ?>
 <?php endif ?>
                         </div>
                       </td>