Macro 32 Ramblings

Mind Archive

Multicast 32:1 Overlapping Addresses


Multicast address always fall in 224 to 239. It means first four bits will be reserved for multicast address. Consequence 28 bits left behind. For the conversation of multicast ip address to mac-address 0100.5e is reserved. So we are left with 24 bits and lost 4 bits during the conversation from ip to mac. 1 bit is used by some other purpose that was being purchased by some. I don’t think so it is really story but a hear sound only. So we can say are only left with 23 bits. 5 bits are not available during the copy of multicast address to hardware or mac-address. 32 multicast IP addresses that map to the MAC address 0x0100.5e01.1020. That’s during the campus design of multicast it is always said that the overlapping of address should kept in mind.





Do not use x.0.0.x or x.128.0.x group addresses



Multicast addresses in the 224.0.0.x range are considered link local multicast addresses. They are used for protocol discovery and are flooded to every port. For example, OSPF uses and for neighbor and DR discovery. These addresses are reserved and will not be constrained by IGMP snooping. Do not use these addresses for an application. Further, since there is a 32:1 overlap of IP Multicast addresses to Ethernet MAC addresses as already explained, any multicast address in the and ranges should NOT be considered.