P2P Programming in Java
The simplest form of P2P programming in Java employs UDP multicasting, which is datagram packets with a MulticastSocket object. A MulticastSocket requires a multicast IP address (a class D IP address) and a port number. Class D IP addresses fall in the range 224.0.0.0 to 239.255.255.255, though certain addresses are reserved.
A peer wishing to subscribe to a multicast group must create a MulticastSocket representing the group and use joinGroup() to begin receiving communication. A peer leaves a group by calling leaveGroup() or by terminating.
Tip
Currently, applets aren't allowed to use multicast sockets.
The application described here takes a (welcome) break from accessing/modifying high score lists, which doesn't make for a particularly suitable P2P example. Instead, a MultiTimeServer
transmits a packet to a multicast group every second, containing the current time and date. MultiTimeClient objects wait for packets to appear in the group and to print them to standard output. The situation is shown in Figure 29-16.

Figure 29-16. UDP multicasting clients and server
The use of the words "client" and "server" are a little misleading since all the objects involved in the group can potentially send and receive messages. It's my choice to restrict the server to sending and the clients to reading.
Tip
The code for the multicasting client and server can be found in the ...