pfSense

/etc/inc/openvpn.inc

The openvpn manual says:
Note: Using - -topology subnet changes the interpretation of the
arguments of - -ifconfig to mean "address netmask", no longer "local
remote".

And also says:
TUN devices in - -topology subnet mode (which create virtual
"multipoint networks"), - -ifconfig is used to set an IP address
and subnet mask ...
(The manual example also shows it.)

But openvpn.inc when using tun still sets ifconfig (conf option)
using $ip1 for client and $ip2 for server instead of the $mask.
I didn't test this but doesn't follow the docs.
This may need fixed so second argument is the mask.

I did read
https://forum.pfsense.org/index.php?topic=103331.0

(I noticed this while working on my book.
My list of bugs is at http://reedmedia.net/books/pfsense/bugs.html )