I don't even understand the network code all that much (actually written by a friend) but it does use INADDR_ANY, INADDR_BROADCAST and SO_BROADCAST.
edit: Tried it on two old Windows 7 computers on my mixed rj45 / wifi home network and some chat data are received by the other party and some don't, or it crashes. Fun.
edit2: Actually works great on a version where I could set a specific broadcast address like 192.168.1.255.
Should work fine on switched network (like DHCP) ? - but won't cross subnets.
Ed: maybe some of you moved to wireless - and some stayed on a different cabled subnet?