Installing and Administering Internet Services

392 Chapter9
Configuring mrouted
Overview of Multicasting
packet, is sent through the intervening, non-multicast 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.
Figure 9-1 Tunnel Made with mrouted Routers
IP Multicast Addresses
IP internet addresses are 32-bit addresses. Each host on the internet is
assigned a unique IP address. There are four classes of IP addresses,
identified as Class A, Class B, Class C, and Class D.Class DIP addresses
are identified as IP multicast addresses. Class A, Class B, and Class C IP
addresses are composed of two parts, a netid (network ID) and a hostid
(host ID). Class D IP addresses are structured differently and are of the
form:
Figure 9-2 Class D IP multicast address format
Bits 0 through 3 identify the address as a multicast address. Bits 4
through 31 identify themulticast group. Multicast addressesare 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, on a local
network, all hosts that participate in IP multicast. The addresses of
non-multicast
R1
tunnel
multicast
transmitter
multicast
recipient
node
node
DVMRP tunnel
endpoint
DVMRP tunnel
endpoint
R2
M
N
router
router
1110
0123
31
Multicast Group Address
4