In order to get the app working you must set a route for the breadcast
address. LANChat uses broadcast 255.255.255.255 and not the usual
broadcast of your subnet. The only thing you need to do is:

/sbin/route add -host 255.255.255.255 eth0

where eth0 is the network interface that communicates with the other
computers running LANChat. If you have 2 network cards you have to check
whether it is eth0 or eth1 or some different name.

(on Slackware Linux the best place to add this route is in /etc/rc.d/rc.inet1)
