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Multicast - an explanation

Since IP is not a reliable network protocol, a new reliable multicast protocol that works at the transport layer and uses IP at the network layer has been developed. It is called Multicast Transport Protocol (MTP)Network interface cards are usually programmed to listen for three types of messages. They are messages sent to their specific address, messages broadcast to all NICs, and messages that qualify as a multicast for the specific card. There are three types of addressing:
  • Unicast - A transmission to a single interface card.
  • Multicast - A transmission to a group of interface cards on the network.
  • Broadcast - A transmission to all interface cards on the network. RFC 919 and 922 describe IP broadcast datagrams.
  • Limited Broadcast - Sent to all NICs on the some network segment as the source NIC. It is represented with the 255.255.255.255 TCP/IP address. This broadcast is not forwarded by routers so will only appear on one network segment.
  • Direct broadcast - Sent to all hosts on a network. Routers may be configured to forward directed broadcasts on large networks. For network 192.168.0.0, the broadcast is 192.168.255.255.
Multicasting may be used for streaming multimedia, video conferencing, shared white boards and more as the internet grows. Multicasting is still new to the internet and not widely supported by routers. New routing protocols are being developed to enable multicast traffic to be routed. Some of these routing protocols are:
  • Hierarchical Distance Vector Multicast Routing Protocol (HDVMRP)
  • Multicast Border Gateway
  • Protocol Independent Multicast
Since IP is not a reliable network protocol, a new reliable multicast protocol that works at the transport layer and uses IP at the network layer has been developed. It is called Multicast Transport Protocol (MTP)

Ethernet Addressing:
The internet assigned numbers authority (IANA) allocates ethernet addresses from 01:00:5E:00:00:00 through 01:00:5E:7F:FF:FF for multicasting. This means there are 23 bits available for the multicast group ID.

IP Addressing:
An IP multicast address is in the range 224.0.0.0 through 239.255.255.255. In hexadecimal that is E0.00.00.00 to EF.FF.FF.FF. To be a multicast address, the first three bits of the most significant byte must be set and the fourth bit must be clear. In the IP address, there are 28 bits for multicasting. Therefore there are 5 multicasting bits that cannot be mapped into an ethernet data packet. The 5 bits that are not mapped are the 5 most significant bits.