Cisco Systems RJ-45-to-AUX manual IP Multicasting Addresses, Multicast IP Structure, 140

each other and with the server by multicast. When an individual driving a tank or a fighter enters a grid square, it joins the multicast group to receive that square’s simulation traffic. As the individual moves from square to square, the individual’s multicast group membership changes.

IP multicasting uses Class D network addresses to route data to different groups and hosts. Most basic networking classes cover Classes A, B, and C, but usually don’t go into depth about Classes D and E. Maybe you were told that they are used for experimental purposes and not to worry about them. Well, as you will see in the next few sections, things have changed.

IP Multicasting Addresses

IP multicasting uses a variation of a Class D network address space assigned by the Internet Assigned Number Authority (IANA). A Class D address is denoted with a binary 1110 prefix in the first octet. The range spans from 224.0.0.0 to 239.255.255.255. Applications use one of the addresses in this range as the multicast group address on the Internet.

Note A permanent IP multicast address is rarely assigned to an application. Instead, the address is assigned to

a specific network protocol or network application. Applications on the Internet must dynamically request a multicast address when needed and release the address when it is no longer being used.

Due to the nature of multicast addresses, they are frequently referred to as multicast groups. In addition, certain well−known groups have been identified by the IANA. These are detailed in RFC 2365 and are known as administrative scopes. We will discuss this topic later in this section.

The Multicast IP Structure

An IP multicast frame contains a single IP address. The host group is also identified by a single IP address. This process operates at Layer 3. However, switches do not understand IP addresses; they operate with Media Access Control (MAC) addresses. Let’s look at how the Layer 3 address is mapped to a Layer 2 address.

IP MAC addresses are 48 bits long. If the first 24 bits are set to 01−00−5e, with the next bit set to 0, that leaves 23 bits for the IP address to be mapped to the MAC address. The multicast IP address is 32 bits long with 28 unique bits. So, only the last 23 bits of the IP address are mapped to the MAC address.

Figure 7.4 shows the MAC address and IP address mapping.

Figure 7.4: The IP multicast address structure.

Let’s take a quick look at how this process works. Consider the multicast address 224.138.8.5:

1110 0000 1000 1010 0000 1000 0000 0101 XXXX XXXX X000 1010 0000 1000 0000 0101

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