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The mrouted Routing Daemon
Chapter 1 19
In this figure, the mrouted router R1 receives a multicast packet from
node M. Because R1 is configured as one end of a tunnel, R1
encapsulates the IP multicast packet in a standard unicast IP packet
addressed to the mrouted router R2. The packet, now treated as a
normal IP packet, is sent through the intervening nonmulticast network
to R2. R2 receives the packet and removes the outer IP header, thereby
restoring the original multicast packet. R2 then forwards the multicast
packet through its network interface to node N.
IP Multicast Addresses
An IP Internet address can be either a 32-bit or a 128-bit address. Each
host on the Internet is assigned a unique IP address. There are four
classes of IP addresses: Class A, Class B, Class C, and Class D. Class D
IP addresses are identified as IP multicast addresses. Class A, Class B,
and Class C IP addresses are composed of two parts: a network ID
(netid) and a host ID (hostid). Class D IP addresses are structured
differently, as shown in Figure 1-2.
Figure 1-2 Class D IP Multicast Address Format
The first 4 bits (0 through 3) identify the address as a multicast address.
Bits 4 through 31 identify the multicast group. Multicast addresses are
in the range 224.0.0.0 through 239.255.255.255. Addresses 224.0.0.0
through 224.0.0.255 are reserved, and address 224.0.0.1 is permanently
assigned to the all hosts group. The all hosts group is used to reach all
the hosts on a local network that participate in IP multicasting. The
addresses of other permanent multicast groups are published in RFC
1060 (Assigned Numbers, March 1990).
You can use IP multicast addresses only as destination addresses, and
they must never appear in the source address field of a datagram.
Internet Control Message Protocol (ICMP) error messages are not
generated for multicast datagrams.
Because IP Internet addressing is a software manifestation of the
underlying physical network, you must map IP addresses to physical
addresses that the hardware comprising the network understands.
1110
0123
31
Multicast Group Address
4