Every IP multicast group has a group address. IP multicast 
provides only open groups: That is, it is not necessary to 
be a member of a group in order to send datagrams to the 
group. 

Multicast address are like IP addresses used for single 
hosts, and is written in the same way: A.B.C.D. Multicast 
addresses will never clash with host addresses because a 
portion of the IP address space is specifically reserved 
for multicast. 

This reserved range consists of addresses from 224.0.0.0 to 
239.255.255.255. However, the multicast addresses from 
224.0.0.0 to 224.0.0.255 are reserved for multicast routing 
information; Application programs should use multicast 
addresses outside this range.