pfSense

Firewall->Aliases, add an alias, type Networks.
Put a range like 192.168.1.10-192.168.1.0
Save

It tries for a while then displays "500 Internal Server Error"

System log has:
Oct 21 14:32:54 lighttpd¹⁸⁹²⁵: (mod_fastcgi.c.3346) response not received, request sent: 1316 on socket: unix:/tmp/php-fastcgi.socket-1 for /firewall_aliases_edit.php?, closing connection
Oct 21 14:32:54 kernel: pid 54376 (php), uid 0: exited on signal 11 (core dumped)
Oct 21 14:32:54 lighttpd¹⁸⁹²⁵: (mod_fastcgi.c.2562) unexpected end-of-file (perhaps the fastcgi process died): pid: 22513 socket: unix:/tmp/php-fastcgi.socket-1

I guess the code in ip_range_to_subnet_array goes into a spin when the range is backward.

It just needs some validation to stop silly stuff like that - I will look at it.