Correct Network IP addressing if your users have ability to VPN in

Techspot has A List of Common Default Router IP Addresses that helps with this. Usually home routers uses /24 subnets. Nowadays mobile phones are often used for sharing network connection, so we must take these ranges into account, too. According to the list we can deduce we should avoid:

• 192.168.0.0/19 - most of the routers seems to use some of these, above 192.168.31.255.
• 10.0.0.0/24 is also widely used, and Apple uses 10.0.1.0/24.
• 192.168.100.0/24 is used by Motorola, ZTE, Huawei and Thomson.
• Motorola uses (in addition) 192.168.62.0/24 and 192.168.102.0/24.
• 192.168.123.0/24 is used by LevelOne, Repotec, Sitecom and U.S. Robotics (less common)
• Some D-Links have 10.1.1.0/24 and 10.90.90.0/24.

We have three ranges reserved for private networks; we still have plenty of space to avoid these in:

• 10.0.0.0/8
• 172.16.0.0/12
• 192.168.0.0/16

Some random upper range from 10.0.0.0/8 could be the safest choice for avoiding collisions. You may also want to avoid number 42 in any part of the IP address range: it might be the most common "random" number, as it's the Answer to the Ultimate Question of Life, The Universe, and Everything.