The beginning of the
Class D range of
IP multicast addresses.
IP multicast addresses, also known as group addresses, are in the Class D range of 224.0.0.0 to 239.255.255.255 as defined by setting the first four high order bits to 1110. In network prefix or Classless Inter-Domain Routing (CIDR) notation, IP multicast addresses are summarized as 224.0.0.0/4. IP Multicast addresses in the range 224.0.0.0 to 224.0.0.255 (224.0.0.0/24) are reserved for the local subnet and are not forwarded by IP routers regardless of the Time to Live (TTL) in the IP header. The following are examples of reserved IP multicast addresses:
Elaboration from http://www.microsoft.com/technet/columns/cableguy/cg0202.asp