Information Technology Reference
In-Depth Information
Multicast IP
Decimal: 239.192.44.56
Hex: EF C0 2C 38
Binary:
11101111 1100000000101100 00111000
Base MAC address
Hex:
01 00 5E 00 00 00
Binary:
00000001 00000000 01011110 00000000 00000000 00000000
Multicast MAC address
Binary:
00000001 00000000 01011110 0100000 000101100 00111000
Hex:
01 00 5E 40 2C 38
Figure 11-24
Mapping of Multicast IP Addressing to MAC Addresses
IGMP
Internet Group Management Protocol is the protocol used in multicast implementations
between the end hosts and the local router. RFC 2236 describes IGMP Version 2
(IGMPv2). RFC 3376 describes IGMP Version 3 (IGMPv3). RFC 1112 describes the first
version of IGMP.
IP hosts use IGMP to report their multicast group memberships to routers. IGMP mes-
sages use IP protocol number 2. IGMP messages are limited to the local interface and are
not routed.
IGMPv1
The first RFC describing IGMP (RFC 1112), written in 1989, describes the host exten-
sions for IP multicasting. IGMPv1 provides simple message types for communication be-
tween hosts and routers. These messages are
Membership query:
Sent by the router to check whether a host wants to join a mul-
ticast group
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Membership report:
Sent by the host to join a multicast group in the segment
The problem with IGMPv1 is the latency involved for a host to leave a group. With
IGMPv1, the router sends membership queries periodically; a host must wait for the mem-
bership query message to leave a group. The query interval is 60 seconds, and it takes
three query intervals (3 minutes) for a host to leave the group.
IGMPv2
IGMPv2 improves on IGMPv1 by allowing faster termination or leaving of multicast groups.
IGMPv2 has three message types, plus one for backward compatibility:
Membership query:
Sent by the router to check whether a host wants to join a group.
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