Java - Networkless UDP broadcast

UDP broadcast works by broadcasting on a port.

import java.net.DatagramSocket;
import java.net.DatagramPacket;


class Broadcast{
    public static void main(String[]){
        DatagramSocket udp = new DatagramSocket(137);
        udp.setbroadcast(true);
        //And so so....

But how can we do this networkless? Routers can do this when computers need an internet connection.