Multicast IP address over LAN - Java Program

Multicast IP address over LAN - Java Program

I am creating a chatting application on peer to peer network. i am trying
to write a java code that will show me on which PC my application is
installed and if that application is running it will show the online like
IP Messenger users. I thought that program will multicast the ip address
on which that program is running and others will detect that multicast ip
and add them to the list for displaying online users..
Please tell me how to write the program for this.
or tell me any other logic for this application to show on which pc my
application is running
thankssss