Devices find each other by NSD (aka Bonjour or ZeroConf), no any configuration required. It just works.
NSD on Android is not stable enough, so if devices can't find each others or top line with pointless text is not green try to restart application, or reboot device.
Program released under GPL license, source code available at https://github.com/js-labs/WalkieTalkie
If you found some bugs or miss some features - contact us.