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Unformatted text preview: ese well-known
multicast addresses are listed in the latest Assigned Numbers RFC. Notice that it is the
multicast address of the group that is permanent, not the membership of the group.
For example, 184.108.40.206 means "all systems on this subnet," and 220.127.116.11 means "all
routers on this subnet." The multicast address 18.104.22.168 is for NTP, the Network Time
Protocol, 22.214.171.124 is for RIP-2 (Section 10.5), and 126.96.36.199 is for SGI's (Silicon
Graphics) dogfight application.
Converting Multicast Group Addresses to Ethernet Addresses
The IANA owns an Ethernet address block, which in hexadecimal is 00:00:5e. This is
the high-order 24 bits of the Ethernet address, meaning that this block includes addresses
in the range 00:00:5e:00:00:00 through 00:00:5e:ff:ff:ff. The IANA
allocates half of this block for multicast addresses. Given that the first byte of any
Ethernet address must be 01 to specify a multicast address, this means the Ethernet
addresses corresponding to IP multicasting are in the range 01:00:5e:00:00:00
Our notation here uses the Internet standard bit order, for a CSMA/CD or token bus network, as the bits
appear in memory. This is what most programmers and system administrators deal with. The IEEE
documentation uses the transmission order of the bits. The Assigned Numbers RFC gives additional
details on the differences between these representations. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/broadcas.htm (9 of 12) [12/09/2001 14.47.00] Chapter 12. Broadcasting and Multicasting This allocation allows for 23 bits in the Ethernet address to correspond to the IP multicast
group ID. The mapping places the low-order 23 bits of the multicast group ID into these
23 bits of the Ethernet address. This is shown in Figure 12.3.
Since the upper 5 bits of the multicast group ID are ignored in this mapping, it is not
unique. Thirty-two different multicast group IDs map to each Ethernet address. For
example, the multicast addresses 188.8.131.52 (hex e0.80.40.2...
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- Spring '12