diff --git a/src/www/interfaces.php b/src/www/interfaces.php
index 989f01bb35..68b82b5bfb 100644
--- a/src/www/interfaces.php
+++ b/src/www/interfaces.php
@@ -248,17 +248,38 @@ function validate_track6_idassoc6(&$pconfig, $if)
         if ($ipv6_delegation_length >= 0) {
             $ipv6_num_prefix_ids = pow(2, $ipv6_delegation_length);
             $track6_prefix_id = intval($pconfig["{$pconfig['type6']}-prefix-id--hex"], 16);
+            $track6_prefix_range = $pconfig["{$pconfig['type6']}_prefix_range"];
             if ($track6_prefix_id < 0 || $track6_prefix_id >= $ipv6_num_prefix_ids) {
                 $input_errors[] = gettext("You specified an IPv6 prefix ID that is out of range.");
+            } elseif (strlen($track6_prefix_range)) {
+                if (!ctype_digit($track6_prefix_range)) {
+                    $input_errors[] = gettext("You specified an IPv6 prefix range that is not valid.");
+                } elseif ($track6_prefix_range < 1 || $track6_prefix_id + $track6_prefix_range > $ipv6_num_prefix_ids) {
+                    $input_errors[] = gettext("You specified an IPv6 prefix range that is out of range.");
+                }
             }
             $default_id = interface_dhcpv6_id($pconfig["{$pconfig['type6']}-interface"]);
             $assoc_pd_ref = !empty($pconfig["{$pconfig['type6']}_assoc_pd"]) && ctype_digit($pconfig["{$pconfig['type6']}_assoc_pd"]) ?
                 $pconfig["{$pconfig['type6']}_assoc_pd"] : $default_id;
             foreach (link_interface_to_track6($pconfig["{$pconfig['type6']}-interface"]) as $trackif => $trackcfg) {
+                if ($trackif == $if) {
+                    continue;
+                }
                 $assoc_pd_link = !empty($trackcfg['track6_assoc_pd']) ? $trackcfg['track6_assoc_pd'] : $default_id;
-                if ($trackif != $if && $assoc_pd_ref == $assoc_pd_ref && $trackcfg['track6-prefix-id'] == $track6_prefix_id) {
+                if ($assoc_pd_ref != $assoc_pd_ref) {
+                    continue;
+                }
+                /* the end of non-overlapping intervals needs to specify 0 ... n-1 */
+                $track6_range_end = !empty($track6_prefix_range) ? $track6_prefix_range - 1 : 0;
+                $track6_range_end += $track6_prefix_id;
+                $range_end_link = !empty($trackcfg['track6_prefix_range']) ? $trackcfg['track6_prefix_range'] - 1 : 0;
+                $range_end_link += $trackcfg['track6-prefix-id'];
+                if ($trackcfg['track6-prefix-id'] == $track6_prefix_id) {
                     $input_errors[] = gettext('You specified an IPv6 prefix ID that is already in use.');
                     break;
+                } elseif ($trackcfg['track6-prefix-id'] <= $track6_range_end && $track6_prefix_id <= $range_end_link) {
+                    $input_errors[] = gettext('You specified an IPv6 prefix range that is already in use.');
+                    break;
                 }
             }
             if (isset($config['interfaces'][$pconfig["{$pconfig['type6']}-interface"]]['dhcp6-prefix-id'])) {
@@ -308,6 +329,9 @@ function store_track6_idassoc6(&$new_config, &$pconfig)
     if (isset($pconfig["{$pconfig['type6']}_ifid--hex"]) && ctype_xdigit($pconfig["{$pconfig['type6']}_ifid--hex"])) {
         $new_config['track6_ifid'] = intval($pconfig["{$pconfig['type6']}_ifid--hex"], 16);
     }
+    if (!empty($pconfig["{$pconfig['type6']}_prefix_range"])) {
+        $new_config['track6_prefix_range'] = $pconfig["{$pconfig['type6']}_prefix_range"];
+    }
     if (!empty($pconfig["{$pconfig['type6']}_assoc_pd"])) {
         $new_config['track6_assoc_pd'] = $pconfig["{$pconfig['type6']}_assoc_pd"];
     }
@@ -504,6 +528,7 @@ if ($_SERVER['REQUEST_METHOD'] === 'GET') {
         'track6-prefix-id',
         'track6_assoc_pd',
         'track6_ifid',
+        'track6_prefix_range',
     ];
     foreach ($std_copy_fieldnames as $fieldname) {
         $pconfig[$fieldname] = isset($a_interfaces[$if][$fieldname]) ? $a_interfaces[$if][$fieldname] : null;
@@ -524,7 +549,7 @@ if ($_SERVER['REQUEST_METHOD'] === 'GET') {
     $pconfig['dhcpd6track6allowoverride'] = isset($a_interfaces[$if]['dhcpd6track6allowoverride']);
     $pconfig['dhcp6_request_dns'] = empty($pconfig['dhcp6_norequest_dns']);
 
-    foreach(['-interface', '-prefix-id', '-prefix-id--hex', '_assoc_pd', '_ifid', '_ifid--hex'] as $fieldname) {
+    foreach(['-interface', '-prefix-id', '-prefix-id--hex', '_assoc_pd', '_ifid', '_ifid--hex', '_prefix_range'] as $fieldname) {
         /* only for form consistency */
         $pconfig["idassoc6{$fieldname}"] = $pconfig["track6{$fieldname}"];
     }
@@ -2641,6 +2666,16 @@ include("head.inc");
                             </div>
                           </td>
                         </tr>
+                        <tr>
+                          <td><a id="help_for_idassoc6_prefix_range" href="#" class="showhelp"><i class="fa fa-info-circle"></i></a> <?=gettext('Reserved prefix range') ?></td>
+                          <td>
+                            <input name="idassoc6_prefix_range" type="text" class="form-control" id="idassoc6_prefix_range" placeholder="1" value="<?= $pconfig['idassoc6_prefix_range'] ?>" />
+                            </div>
+                            <div class="hidden" data-for="help_for_idassoc6_prefix_range">
+                              <?= gettext('The value in this field is the length of the reserved prefix range for downstream prefix delegation. The range starts at the given prefix ID. The default is to only reserve the given prefix ID.') ?>
+                            </div>
+                          </td>
+                        </tr>
                         <tr>
                           <td><a id="help_for_idassoc6_ifid" href="#" class="showhelp"><i class="fa fa-info-circle"></i></a> <?= gettext('Optional interface ID') ?></td>
                           <td>
@@ -2706,6 +2741,16 @@ include("head.inc");
                             </div>
                           </td>
                         </tr>
+                        <tr>
+                          <td><a id="help_for_track6_prefix_range" href="#" class="showhelp"><i class="fa fa-info-circle"></i></a> <?=gettext('Reserved prefix range') ?></td>
+                          <td>
+                            <input name="track6_prefix_range" type="text" class="form-control" id="track6_prefix_range" placeholder="1" value="<?= $pconfig['track6_prefix_range'] ?>" />
+                            </div>
+                            <div class="hidden" data-for="help_for_track6_prefix_range">
+                              <?= gettext('The value in this field is the length of the reserved prefix range for downstream prefix delegation. The range starts at the given prefix ID. The default is to only reserve the given prefix ID.') ?>
+                            </div>
+                          </td>
+                        </tr>
                         <tr>
                           <td><a id="help_for_track6_ifid" href="#" class="showhelp"><i class="fa fa-info-circle"></i></a> <?= gettext('Optional interface ID') ?></td>
                           <td>