Bonjour Zeroconf Chat

Bonjour is Apple's implementation of Zeroconf. It discovers other nodes by broadcasting special DNS packets on the local network using IP Multicast addresses in the process. As frequently with so called multicast applications in LANs, those addresses are not routed to the outer Internet, thus no multicast routing actually takes place. It's just a coincidence that addresses originally reserved for multicasting became popular for local broadcasting instead.

Bonjour chat clients use Zeroconf to detect communication partners in the LAN, then use XMPP to exchange IM with them.