Allied Telesis AR400 manual Choosing group addresses, IP address, binary, MAC address, hex

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 IP addresses in the range 224.0.0.0-239.255.255.255 are multicast addresses, but many addresses 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 Another complication is that multicasting is designed to use each packet’s group IP address to same MAC 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 01-00-5E.

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.128.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, 224.128.6.200, 225.0.6.200, 225.128.6.200, etc.

To see this in detail, consider 224.0.6.200. This has a multicast MAC of 01-00-5E-00-06-C8, like this:

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

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:	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

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