Administrative Distance, Access Lists, Route Manipulation, and IP Multicast - CCIE Routing and Switching Exam Certification Guide - page 490

that source and receivers for multicast applications do not use. Also, the administrative scoped
addresses do not cross administrative boundaries of multicast networks.

Layer-3 to Layer-2 Mapping
Multicast-aware Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI) network
interface cards use the reserved IEEE 802 address 0100.5e00.0000 for multicast addresses at
the MAC layer. In the high order byte, 0x01, the low order bit set to 1. This is the I/G bit and
signifies whether the address is an individual address (0) or a group address (1). This bit is set
to 1 for multicast addresses.

Ethernet interfaces map the lower 23 bits of the IP multicast address to the lower 23 bits of the
MAC 0100.5e00.0000. As an example, the IP multicast address 224.0.0.2 is mapped to the
MAC layer as 0100.5e00.0002. Figure 10-4 shows another example that looks at the bits of
multicast IP 236.130.44.56. The IP address in hexadecimal is EC:82:2C:38. The lower 23 bits
are mapped into the lower 23 bits of the base Multicast MAC to produce the multicast MAC
address of 01:00:5E:02:2C:38.

Mapping of Multicast IP Addressing to MAC Addresses Figure 10-4

Multicast IP
Decimal: 236.130.44.56

Hex: EC

82 2C 38

Binary: 11101100 10000010 00101100 00111000

Base MAC address
Hex: 00 00 00
Binary: 00000001 00000000 01011110 00000000 00000000 00000000

01 00 5E

Multicast MAC address
Binary: 00000001 00000000 01011110 00000010 00101100 00111000
Hex:

38

01 00 2C

5E 02

IGMP

IGMP is the protocol that multicast implementations use between end hosts and the local router.
IGMP version 2 (IGMPv2) is described in RFC 2236. The first version of IGMP is described
in RFC 1112.

IP hosts use IGMP to report their multicast group memberships to routers. IGMP messages use
IP protocol number 2. IGMP messages are limited to the local interface and are not routed.