FROM:http://chaos.weblogs.us/archives/164
Python has complete support for sockets. Some of the APIs though have a different signature than the POSIX equivalents. It is therefore possible to write multicast servers and clients in Python. Multicasting is the ability to send data to select set of hosts. Broadcasting in a network is to send data to all the hosts, unicast is when data is transfered between two hosts (typical one-one communication). Multicasting is the ability to send data to a multicast address and the clients are provided the data. Now that an introduction to multicasting is complete, lets look at how to write a simple multicast server and a multicast client.
Multicast servers are written very similar to a standard unicast server, its the clients who have a slightly different code. The major change is made in setting socket options on the socket transmitting and receiving data.

The server code looks like this:
################################################################
import socket, time

ANY = "0.0.0.0"
SENDERPORT = 1501
MCAST_ADDR = "224.168.2.9"
MCAST_PORT = 1600

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP)
#The sender is bound on (0.0.0.0:1501)
sock.bind((ANY,SENDERPORT))
#Tell the kernel that we want to multicast and that the data is sent to everyone (255 is the level of multicasting)
sock.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_TTL, 255)
while 1:
    time.sleep(10)
    #send the data "hello, world" to the multicast addr: port
#Any subscribers to the multicast address will receive this data
    sock.sendto("Hello World", (MCAST_ADDR,MCAST_PORT));
################################################################
The TTL field of 255 indicates that the data is unrestricted in scope.

The corresponding client code to receive the data being transmitted by the sender will look like this:
################################################################
import socket, time

ANY = "0.0.0.0"
MCAST_ADDR = "224.168.2.9"
MCAST_PORT = 1600
#create a UDP socket
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP)
#allow multiple sockets to use the same PORT number
sock.setsockopt(socket.SOL_SOCKET,socket.SO_REUSEADDR,1)
#Bind to the port that we know will receive multicast data
sock.bind((ANY,MCAST_PORT))
#tell the kernel that we are a multicast socket
sock.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_TTL, 255)
#Tell the kernel that we want to add ourselves to a multicast group
#The address for the multicast group is the third param
status = sock.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP,
    socket.inet_aton(MCAST_ADDR) + socket.inet_aton(ANY));

sock.setblocking(0)
ts = time.time()
while 1:
    try:
        data, addr = sock.recvfrom(1024)
    except socket.error, e:
        pass
    else:
        print "We got data!"
        print "FROM: ", addr
        print "DATA: ", data
################################################################
The import part of the client code is the setsockopt for multicast socket and to add the socket to a multicast group (socket.IP_ADD_MEMBERSHIP). Also note that the socket is bound to ANY:MCAST_PORT. The server sends data to the multicast address and on the client side, the socket that is bound to the multicast port subscribes to the multicast group and receives data on the MCAST_PORT. It is important the client code bind itself to the MCAST_PORT to receive data from the multicast address. Another important thing to note is the MCAST_ADDR. This address should be a valid multicast address, and not a unicast (localhost / INADDR_ANY / any valid IP).

TTL     Scope
----------------------------------------------------------------------
   0    Restricted to the same host. Won't be output by any interface.
1    Restricted to the same subnet. Won't be forwarded by a router. <If not otherwise specified, multicast datagrams are sent with a default value of 1, to prevent them to be forwarded beyond the local network.
 <32         Restricted to the same site, organization or department.
 <64 Restricted to the same region.
<128 Restricted to the same continent.
<255 Unrestricted in scope. Global.

224.0.1.0到238.255.255.255为用户可用的组播地址，在全网范围内有效。其中232.0.0.0/8为SSM组地址，而其余则属于ASM组地址。

http://www.h3c.com.cn/Products___Technology/Technology/Group_Management/Other_technology/Technology_book/200803/336048_30003_0.htm
根据接收者对组播源处理方式的不同，组播模型分为以下两大类：
ASM模型：即任意源组播模型。在ASM模型中，任一发送者都可作为组播源向某组播组地址发送组播信息，接收者通过加入由该组播组地址标识的组播组以获得发往该组播组的组播信息。在ASM模型中，接收者无法预先知道组播源的位置，但可以在任意时间加入或离开组播组。SSM模型：即指定信源组播模型。在现实生活中，用户可能只对某些组播源发送的组播信息感兴趣，而不愿接收其它源发送的信息。SSM模型为用户提供了一种能够在客户端指定组播源的传输服务。