Multicasting

    Multicasting is a term used for streaming data to multiple destinations at the same time. It's an efficient way to send high bandwidth data, such as multimedia, when you have a large number of users who want the same data.

    The usual way data is sent over the Internet is through unicasting. For example, with unicast each user who wants to view a movie gets his or her own copy transmitted from the server. If one thousand users request to view the same movie, then one thousand copies of the movie are transmitted over the Internet. With multicasting, only one copy of the movie is transmitted, but any user who wants to view it can tune in to the transmission.

    Because the Internet is set up to do unicasting and not multicasting, several protocols and technologies have been created to allow multicasts to be transmitted over the Internet:
    Multicast IP addresses. A range of IP addresses has been set aside as multicast addresses. The range is 224.0.0.0 to 239.255.255.255.
    Multicast Backbone (MBONE). The MBONE is a virtual network that supports IP multicasting. It uses the same physical media as the Internet, but it's designed to "repackage" multicast data packets so they appear to be unicast data packets.
    Tunneling. This is the scheme of hiding multicast packets so that unicast routers will transmit the packets.
    Multicast routers. These are devices that can recognize multicast packets and transmit the data properly.

    For more information about multicasting, see these sites:
    IP Multicast Initiative Web site
    www.ipmulticast.com/
    QuickTime Web site
    www.apple.com/quicktime/

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