transparent-gateway.rst: Add details on the network configuration

This is a first stab at documenting the network properties used in the
gateway blueprint. It reflects the current status and needs to be kept
in sync with changes.

Fixes: https://git.ostc-eu.org/OSTC/planning/core-os/-/issues/201



Signed-off-by: Stefan Schmidt <stefan.schmidt@huawei.com>

docs/blueprints/transparent-gateway.rst

@@ -15,9 +15,45 @@ The Transparent Gateway Blueprint provides support for building a PoC gateway th
 
 #. Ability to participate in an OpenThread network
 #. Automatic IPv6-to-IPv4 translation between devices and rest of the world
-#. Wifi AP functionality
+#. WiFi AP functionality
 #. Secure OTA
 
+
+Network Subnets and Configuration
+*********************************
+
+Depending on the used board and hardware configuration, the available network interfaces, and their names, can vary. For the sake of this blueprint, we assume the following interfaces being available::
+
+   Ethernet interface eth0: assumed to be uplink with DHCP enabled
+   WiFi interface wlan0: WiFi access point interface serving the WiFi subnet
+   OpenThread interface wpan0: OpenThread Border Router interface serving the mesh network
+
+In terms of IP subnets, we are using the private 172.16.47.0/24 range on the WiFi subnet. The AP itself has 172.16.47.1/24 assigned. Clients are being served DHCP leases in the range 172.16.47.100 - 172.16.47.150. The default DNS servers are 9.9.9.9 as primary and 8.8.8.8 as secondary. For IPv6, we rely on address auto-configuration for the time being.
+
+On the OpenThread mesh network subnet, no IPv4 is available, and again we rely on address auto-configuration for the time being.
+
+Forwarding for IPv4 and IPv6 is enabled on all interfaces with sysctl.
+
+WiFi Access Point Configuration
+*******************************
+
+In our default WiFi access point configuration, we create an AP on channel 6 in the 2.4 GHz band with WPA2 pre-shared key configuration::
+
+   SSID: "All Scenarios OS WiFi"
+   Passphrase: "12345678".
+
+For more details, the used `hostapd configuration file <https://git.ostc-eu.org/OSTC/OHOS/meta-ohos/-/blob/develop/meta-ohos-blueprints/recipes-connectivity/hostapd/files/hostapd.conf>`_ is the best reference.
+
+OpenThread Border Router Configuration
+**************************************
+
+In our default OpenThread Border Router configuration, we create an OpenThread mesh network on channel 26 in the 2.4 GHz band with panid 0x1357::
+
+   Networkname "ASOS Thread"
+   OpenThread masterkey: 00112233445566778899aabbccddeeff
+
+For more details, the used `OpenThread configuration script <https://git.ostc-eu.org/OSTC/OHOS/meta-ohos/-/blob/develop/meta-ohos-blueprints/recipes-connectivity/openthread/ot-br-posix/otbr-configuration>`_ is the best reference.
+
 Get sources
 ***********
 #.