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)

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
192.168.85 # 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