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Page 7 | AlliedWare™ OS How To Note: IGMP
IGMP overview > Choosing group addresses
Choosing group addresses
This section describes things you need to be aware of when choosing addresses for your
multicast groups.
Reserved IP
addresses
IP addresses in the range 224.0.0.0-239.255.255.255 are multicast addresses, but many
addresses in this range are reserved. Therefore, before choosing a multicast address, you
should check its status in the “Internet Multicast Addresses” document at the IANA website
at www.iana.org/assignments/multicast-addresses.
IPs using the
same MAC
Another complication is that multicasting is designed to use each packet’s group IP address to
determine a multicast MAC address to send the packet to. However, multicasting does not
have a
1
:
1
mapping of IP address to MAC address—instead each multicast MAC address
corresponds to 32 multicast IP addresses. This means that different multicast IP addresses
use the same MAC address.
The MAC address only uses the last 23 bits of the IP address; it ignores the IP’s first octet and
the first bit of the second octet. Note that all IP multicast MAC addresses start with
0
1
-00-5E.
You need to avoid using multiple IP addresses that have the same MAC address. In practice,
this means that if you use x.0.y.z, then do not use x.
1
28.y.z (or vice versa), where x is
anything from 224-239, and y and z are the same in each IP address. For example, if y=6 and
z=200 then these IP addresses use the same MAC: 224.0.6.200, 224.
1
28.6.200, 225.0.6.200,
225.
1
28.6.200, etc.
To see this in detail, consider 224.0.6.200. This has a multicast MAC of 0
1
-00-5E-00-06-C8,
like this:
Therefore, the following multicast IP addresses will all have the same MAC address as
224.0.6.200, because their last 23 bits are all the same:
IP address, decimal:
224. 0. 6. 200
IP address, binary:
11100000 00000000 00000110 11001000
MAC address, binary:
0000000 00000110 11001000
MAC address, hex:
01-00-5E -00 -06 -C8
IP address, decimal: IP address, binary:
224.0.6.200 11100000 0 0000000 00000110 11001000
224.128.6.200 11100000 1 0000000 00000110 11001000
225.0.6.200 11100001 0 0000000 00000110 11001000
225.128.6.200 11100001 1 0000000 00000110 11001000
226.0.6.200 11100010 0 0000000 00000110 11001000
226.128.6.200 11100010 1 0000000 00000110 11001000
227.0.6.200 11100011 0 0000000 00000110 11001000
227.128.6.200 11100011 1 0000000 00000110 11001000
... ... ...
239.0.6.200 11101111 0 0000000 00000110 11001000
239.128.6.200 11101111 1 0000000 00000110 11001000
Different IPs The same MAC