This is useless for local server that you use for yourself. Not for multiplayer or over network connection!
Edit this file conf/subnet_athena.conf, you can open it with text editor.
// Subnet support file
// Format is:
// subnet: net-submask:char_ip:map_ip
// you can add more than one subnet (max 16)
// check is if((net-submask & char_ip ) == (net-submask & servip)) => ok
subnet: 255.0.0.0:127.0.0.1:127.0.0.1
By using subnet/netmask you can restrict only player from X region that can connect to your server. Of course you need to know the netmask and IP ranges.
This configuration file commonly used for server that running in server host. Example the IP that you get from provider is 213.123.213.123. You need to put this IP in conf/login_athena.conf, conf/char_athena.conf, and conf/map_athena.conf instead of using 127.0.0.1.
And your subnet_athena.conf will be
subnet: 255.0.0.0:213.123.213.123:213.123.213.123
Cydh says: "I know how subnetting works, but I'm not a good teacher!"