Installing and Administering Internet Services

306 Chapter 9

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 D IP 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 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, on a local

non-multicast

tunnel

multicast

transmitter

multicast

recipient

node

DVMRP tunnel

endpoint

DVMRP tunnel

endpoint

router

1110

0123

Multicast Group Address