Send an UDP packet to the broadcast address of your network (all
significant parts set to 255). If this makes no sense to you, you need
to learn about how computer networks work first.

Links found with searching "java network":

<http://www.davidreilly.com/java/java_network_programming/>
<http://java.sun.com/docs/books/tutorial/networking/index.html>

Searching "java network broadcast":

<http://www.developer.com/java/other/article.php/3315501>
<http://java.sun.com/docs/books/tutorial/networking/datagrams/broadcasting.html>
<http://www.java-tips.org/java-se-tips/java.net/how-can-i-send-broadcast-packets-
in-java.html>

and many more.