diff --git a/reference/filter/filters.xml b/reference/filter/filters.xml
index fc45e43b01..5f556a2e91 100644
--- a/reference/filter/filters.xml
+++ b/reference/filter/filters.xml
@@ -694,6 +694,10 @@ filter.default_flags = 0
          <literal>::1/128</literal>, <literal>::/128</literal>,
          <literal>::ffff:0:0/96</literal> and <literal>fe80::/10</literal>.
         </para>
+        <para>
+         These are the ranges that are marked as Reserved-By-Protocol in
+         <link xlink:href="&url.rfc;6890">RFC 6890</link>.
+        </para>
        </entry>
       </row>
       <row>