The IPs in the VPN are assigned randomly. If you have your router configured using something like Tomato, you can setup the reservation by MAC address. For the linux-based install, follow these steps:
1) To begin, first identify the directory, where the files will be stored:
2) Add the path to your /etc/openvpn/server.conf file:
3) Each client has to have it's own IP reservation. It is done based on CN from when each client was generated. Check your .cer or .crt file for a similar Subject line. In our case, the CN is client-connection
4) Edit the file (vim /etc/openvpn/reserved/client-connection) and add your IP (in our case it is 172.16.0.15):
ifconfig-push 172.16.0.15 255.255.255.0
#you can also optionally push the gateway, if you want the traffic to go through VPN. Uncomment below and replace as needed
#push "route 172.16.0.0 255.255.255.0 172.16.0.1"
and you are done.