IP Multicasting Issues

By Chris Koeritz

    This document describes IP multicast and compares it with directed IP broadcast.  It is intended as a brief introduction rather than a technical explanation.  For more information than is available here, you might want to check out this link: http://www.multicasttech.com/faq/.

 Directed IP Broadcast

    The diagram below shows the essential points of a directed broadcast.  The sender transmits some data (denoted by the little envelopes) out to the three subnets depicted.  The data must actually be sent three times, one for each subnet.  On the receiving subnets, every machine on the subnet will be sent the data, whether it wants it or not.


IP Multicast

    The diagram below shows the essential points of a multicast.  The sender transmits some data out to a multicast address.  The data only needs to be sent once, and the routers will take care of the rest.  The middle subnet has no hosts registered for this data and so will not see the data at all.  On the other two subnets, only the hosts that have specifically registered for the data will see it; the other machines on the subnet will not.  Note that the routers manage the delivery list; the sender need not know who the recipients are (and it will not know the recipients unless a higher level protocol is providing the information).


An Assortment of Multicast Issues

  • Multicast Group Addresses
  • Address Allocation
  • Multicast Properties
  • Multicast Scope
  • Comparison With Standard TCP/IP
  • Comparison With Directed IP Broadcast

  • What are the drawbacks of using multicast?
  • Why is multicast a potentially superior choice for data distribution?