Philip Rowlands wrote:
On 19/01/2011 01:52, Marcy Cortes wrote:
Thanks Scott, Philip, and Alan,
That gives me enough info. Curious though how to map the mac addr to
the IP from Linux itself? They don't show up in arp -n . Would one
check at the router?
There's a defined many -> one mapping from IP multicast addresses to MAC
addresses which begin 01:00:5E: (or 33:33: for IPv6).
http://www.tcpipguide.com/free/t_TCPIPAddressResolutionForIPMulticastAddresses.htm
http://en.wikipedia.org/wiki/Multicast_address#Ethernet
You can't map back directly from MAC address to IP though, as some bits
were dropped in the IP->MAC mapping. I assume (speculation follows) that
this scheme helps unburden the kernel by programming the network
hardware with interesting MAC addresses, ignoring otherwise irrelevant
packets.
Official MAC addresses have a global unique portion, the first 24 bits.
To ensure multicasts frames are universally recognizable, a global
unique address was requested and assigned, just as a NIC manufacturer is
required to do. That left the other 24-bits for the locally assigned
portion of the MAC address, these bits being derived from the multicast
IPv4 address. At least that is my understanding of how this scheme
originated.
Harold Grovesteen
Cheers,
Phil
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