diff --git a/isp_box_config.md b/isp_box_config.md
index 376836a6..465a82eb 100644
--- a/isp_box_config.md
+++ b/isp_box_config.md
@@ -32,6 +32,10 @@ Opening the ports listed below is necessary for the various services available i
 * [XMPP](/XMPP_en): 5222 <small>(clients)</small>, 5269 <small>(servers)</small>
 * [Email](/email_en): 25, 587 <small>(SMTP)</small>, 993 <small>(IMAP)</small>
 
+If you use both a modem and a router, then you need to do the following:
+1. first on the modem (the box closest to the internet) create rules to forward the above ports to your router;
+2. then on the router (the box between the modem and your devices) create rules to forward the above ports to the static IP address for your server.
+
 <div class="alert alert-warning" markdown="1">
 <span class="glyphicon glyphicon-warning-sign"></span> Some internet service providers block port 25 (mail SMTP) by default to fight spam. Some other ISP don't allow to use port 80/443 (web) freely, though it's less likely. Depending on the ISP, it might be possible to open them in the admin interface... Check [this page](isp_en) for more info.
 </div>