[Ref Arch] Rephrase router gateway IP description

This patch:
- adds a description of router gateway IP while adding a new router;
- adds pictures that show new router gateway selection;
- adds a reference to the Configuring the network diagram;
- adds the router network IP 12.0.0.1/24 as example to the diagram
  description.

Change-Id: I867cb13b4049b9a27d551170f3152231c499f09c
Closes-Bug: #1421544

pages/reference-architecture/network-concepts/6030-router.rst

@@ -4,11 +4,16 @@
 Router
 ------
 
-Your network must have an IP address set on a router
-in the Public :ref:`logical-networks-arch`.
-Without this, your VMs are unable to access the outside world.
-In many of the examples provided in these documents, 
-that IP is 12.0.0.1 in VLAN 101.
+Your network must have an IP address in the Public :ref:`Logical Networks
+<logical-networks-arch>` set on a router port as an "External Gateway".
+Without this, your VMs are unable to access the outside world. In many
+of the examples provided in these documents, that IP is :ref:`12.0.0.1 in VLAN 101 <conf_netw>`.
+
+If you add a new router, be sure to set its gateway IP:
+
+.. image:: /_images/new_router.png
+
+.. image:: /_images/set_gateway.png
 
 The Fuel UI includes a field on the networking tab for the gateway address.
 When OpenStack deployment starts,

pages/reference-architecture/nova-intro/0630-nova-vlan.rst

@@ -40,12 +40,15 @@ Simplifying the configuration of VLAN Manager,
 there is no known limitation
 that Fuel could add in this particular networking mode.
 
+.. _conf_netw:
+
 Configuring the network
 -----------------------
 
 Once you choose a networking topology (Nova-network FlatDHCP or VLAN),
 you must configure equipment accordingly.
-The diagram below shows an example configuration.
+The diagram below shows an example configuration (with a router network
+IP 12.0.0.1/24).
 
 .. image:: /_images/physical-network.png
   :width: 90%