**Avoid routing conflicts**


The following applies to a routed TUN setup which should just about always be the way to setup OpenVPN.

In order to avoid routing conflicts one should choose subnets carefully for the networks **under ones control**.
These should be RFC 1918 compliant, https://tools.ietf.org/html/rfc1918.

```
10.0.0.0        -   10.255.255.255  (10/8 prefix)
172.16.0.0      -   172.31.255.255  (172.16/12 prefix)
192.168.0.0     -   192.168.255.255 (192.168/16 prefix)
```

![Visualization of OpenVPN routed TUN with subnet topology](./arc1.png "Routed TUN - Topology subnet")

Traveling clients like laptops and mobile phones mostly have no control over the network they reside.
In those cases it becomes even more important to choose **uncommon** subnets.

In the simplest form there are three networks involved, see Figure 1:
1. The Server side subnet
2. The Tunnel subnet
3. The Client side subnet



Figure 1

There may be no overlap between 1, 2 and 3.
Basically, normal routing applies which should now be obvious to the network admin.

The following (incomplete) list is comprised of default settings for routers/acces points/switches/etc.
These are common subnets one should try to avoid:

```
10.0.0
10.0.1
10.1.1
10.1.10
10.2.0
10.8.0
10.10.1
10.90.90
10.100.1
10.255.255

169.254 # APIPA #

172.16.0
172.16.16
172.16.42
172.16.68

172.19.3

172.20.10 # IPhone built-in hotspot #

192.168.0
192.168.1
192.168.2
192.168.3
192.168.4
192.168.5
192.168.6
192.168.7
192.168.8
192.168.9
192.168.10
192.168.11
192.168.13
192.168.15
192.168.16
192.168.18
192.168.20
192.168.29
192.168.30
192.168.31
192.168.33
192.168.39
192.168.40
192.168.42 # Android USB tethering #
192.168.43 # Android built-in hotspot #
192.168.50
192.168.55
192.168.61
192.168.62
192.168.65
192.168.77
192.168.80
192.168.84 # Google WiFi
192.168.85 # Google WiFi
192.168.86 # Google WiFi
192.168.88
192.168.98
192.168.99
192.168.100
192.168.101
192.168.102
192.168.111
192.168.123
192.168.126
192.168.129
192.168.137 # Windows Phone built-in hotspot #
192.168.168
192.168.178
192.168.190
192.168.199
192.168.200
192.168.220
192.168.223
192.168.229
192.168.240
192.168.245
192.168.251
192.168.252
192.168.254

200.200.200
```



November 5 2019 by Pippin