Definition of multicasting in The Network Encyclopedia.

What is Multicasting (in computer networking)?

A technique in which a single copy of data is sent over a network to a specific group of computers. Multicasting is connectionless - clients receive multicast transmissions by “tuning in” to them, a process similar to tuning in to a radio station. Multicasting is essentially a point-to-multipoint communication mechanism.

Microsoft NetShow is an example of an application that uses multicasting technology to transmit streaming audio and video over the Internet and corporate intranets. Multicasting is unidirectional communication between a NetShow server and a NetShow client.

How It Works

NetShow multicasts require two pieces of information:

  • A destination IP address and port number indicating the IP address and port to which the client should “listen” for multicasts
  • Multicast scope, which determines how widely the multicast is distributed over the network

The method used to inform the client of the destination IP address and port depends on the kind of stream being multicast. The types of streams are as follows:

  • Advanced Streaming Format (ASF):
    The destination IP address and port are specified when a channel is created using the NetShow Administrator program. An announcement tells the client how to locate the channel. The channel contains information about which IP address and port the client should listen to, the format of each stream, and the streaming information itself.


  • Real-Time Protocol (RTP) live audio streams, WAV audio streams, and streaming file transfers:
    Information about the destination IP address and port is specified for each stream in the program when you create a program using NetShow Administrator. To receive these multicasts, this information must match the IP address and port specified using the control on the client’s Web page. All the information a client needs to receive the multicast is provided when you include the audio or file transfer client OLE custom control (OCX) on a Web page.


Multicast IP addresses are class D addresses and fall in the range through For intranet use, addresses in the range 239.*.*.* are recommended. Port numbers can range from 1 through 65535, although ports below 1024 are reserved, as are some ports above 1024. Port numbers for RTP audio should be even numbers in the range 16384 through 32768.

In NetShow you set multicast scopes by specifying a Time to Live (TTL) value, which decrements each time packets pass through a router. NetShow default scope values are as follows:

  • Local network = 1
  • Intranet = 32
  • Internet = 128

These values can be customized to meet the needs of your network.