Add Flat mode doc

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+..
+      Copyright 2010 United States Government as represented by the
+      Administrator of the National Aeronautics and Space Administration.
+      All Rights Reserved.
+
+      Licensed under the Apache License, Version 2.0 (the "License"); you may
+      not use this file except in compliance with the License. You may obtain
+      a copy of the License at
+
+          http://www.apache.org/licenses/LICENSE-2.0
+
+      Unless required by applicable law or agreed to in writing, software
+      distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+      WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+      License for the specific language governing permissions and limitations
+      under the License.
+
+
+Flat Network Mode
+=================
+
+Flat network mode removes most of the complexity of VLAN mode by simply
+bridging all instance interfaces onto a single network.
+
+There are two variations of flat mode that differ mostly in how IP addresses
+are given to instances.
+
+
+Original Flat Mode
+------------------
+
+Each compute host creates a single bridge for all instances to use to attach
+to the external network.
+
+The networking configuration is injected into the instance before it is booted or
+it is obtained by a guest agent installed in the instance.
+ 
+Note that the configuration injection currently only works on linux-style systems that keep networking 
+configuration in /etc/network/interfaces.
+
+
+Flat DHCP Mode
+--------------
+
+Like flat mode, all instances are attached to a single bridge on the compute node.
+In addition a DHCP server is running to configure instances.
+
+
+Implementation
+--------------
+
+The network nodes do not act as a default gateway in flat mode.  Instances
+are given public IP addresses.
+
+Compute nodes have iptables/ebtables entries created per project and
+instance to protect against IP/MAC address spoofing and ARP poisoning.
+
+
+Examples
+--------
+
+.. todo:: add flat network mode configuration examples