diff --git a/docs/UPnP.xml b/docs/UPnP.xml
index 7b62fe7fa..df9c43684 100644
--- a/docs/UPnP.xml
+++ b/docs/UPnP.xml
@@ -65,6 +65,12 @@
       arbitrary local systems on any port whatsoever. In short: USE UPnP
       <emphasis role="bold">AT YOUR OWN RISK.</emphasis></para>
     </warning>
+
+    <important>
+      <para>Shorewall and linux-igd implement a UPnP <firstterm>Internet
+      Gateway Device</firstterm>. It will not allow clients on one LAN subnet
+      to access a UPnP Media Server on another subnet.</para>
+    </important>
   </section>
 
   <section id="linux-igd">