OpenVPN Access Server is not Open Source software and as such you will need to
raise a ticket on this system for support:
https://openvpn.net/index.php/login.html
Access Server Docs:
https://openvpn.net/index.php/access-server/docs.html
http://openvpn.net/images/pdf/OpenVPN_Access_Server_Sysadmin_Guide_Rev.pdf
As you are doing this for your personal edification I recommend you try OpenVPN Community Edition.
https://openvpn.net/index.php/download/community-downloads.html
alvaro wroteThe clients that connect with my server always receive a different subnet mask that I've configured at the server side
As I cannot offer any support for your question I will just offer you this .. it may help .. or it may not.
See
--topology in
The Manual v23x