Multicasting offers the ability to send data to more than one
station but not to all stations. It differs from a broadcast in that
a broadcast data will be sent to all stations on the
network.
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Stations can be members of a multicast group.
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Class D addresses are multicast addresses.
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A Class D address will map to a MAC address.
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The first four bits of a Class D IP address are:
1110.
A Class D address expressed in dotted decimal notation will
fall into the following range:
224.0.0.0 through 239.255.255.255
If a station carries a Class D address this identifies the
station as being a member of a multicast group.
The Internet Assigned Numbers Authority (IANA) owns a block
of Ethernet addresses which range from 01:00:5e:00:00:00 through
01:00:5e:7f:ff:ff. These MAC addresses are used for IP
multicasting.
If station A sends a frame to a station B station (and
station B has a Class D address), Station A will take the lower
order 23 bits of the Class D address and insert them into the
MAC-layer destination address.
The most significant 9 bits of the IP address are not
used.
Because these 9 bits are not used in the MAC address it is
possible for the resulting MAC address to point to more than one
station. Therefore it becomes the responsibility of the NIC to
filter out unwanted transmissions.
Refer to the example below:
224.128.64.32 or 0xe0.80.40.20 224.0.64.32 or
0xe0.00.40.20
Both of the above addresses would map to the following MAC
address: 02:00:5e:00:40:20.
IP Address
Decimal
224
128
64
32
Binary
1110 0000
1000 0000
0100 0000
0010 0000
Hex
e 0
8 0
4 0
2 0
The above address will be mapped to the following MAC
address:
The first nine bits of the IP address are not used.
Binary
0000 0001
0000 0000
0101 1110
0000 0000
0100 0000
0001 0000
Hex
0 1
0 1
0 1
0 1
0 1
0 1
Since the first nine bits are not used; the most significant
bit of the 2nd octet of the IP address will be dropped; therefore
there is no way to distinguish whether that bit was set to 1 or 0.
As a result both of the IP addresses would be mapped to the same MAC
address. Even in this scenario it still beats
broadcasting.
Every MAC layer address that begins with 01 will identify a
multicast address.
Some multicast addresses are assigned by the IANA.
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224.0.0.1 identifies all systems on this subnet
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224.0.0.2 identifies all routers on this subnet
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224.0.1.1 is used for Network Time Protocol (NTP)
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224.0.0.9 is for RIP version 2
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224.0.1.2 is for Silicon Graphics Dogfight
Application
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224.0.0.5 identifies routers other than Designated
Routers
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224.0.0.6 identifies Designated Routers (DR)
Internet Group Management Protocol (IGMP) is a protocol that
will let routers know if any hosts on a network belong to a
multicast group.