Albert Manfredi wrote:
> "Andy Furniss" wrote:
>
>> David Tiktin wrote:
>>
>>> If you have a capture, could you tell us what the Ethernet address of
>>> the frame containing the IP multicast packet is? Given the other
>>> comments in this thread, I'm just curious ;-)

>>
>> Here's one, dst mac is not mac of capture PC, src is mac of sender.
>>
>> 00:c0:95:e2:57:04 > 01:00:5e:7f:00:01, ethertype IPv4 (0x0800), length
>> 1358: IP 192.168.0.1.32771 > 239.255.0.1.1234: UDP, length: 1316

>
> Not sure what you're saying, Andy. The destination MAC address is not
> supposed to be the MAC address of the destination PC, in this multicast
> case. It is instead supposed to be a MAC address derived from the IP
> address, following the algorithm in RFC 1112.
>
> I believe 01:00:5e:7f:00:01 *is* that MAC address. RFC 1112, Section 6.4
> states:
>
> An IP host group address is mapped to an Ethernet multicast address
> by placing the low-order 23-bits of the IP address into the low-order
> 23 bits of the Ethernet multicast address 01-00-5E-00-00-00 (hex).
> Because there are 28 significant bits in an IP host group address,
> more than one host group address may map to the same Ethernet
> multicast address.
>
> The source MAC is supposed to be the MAC of the sender or of the router,
> depending if the sender and the receiver are in th same IP subnet or in
> different IP subnets.
>
> RFC 1112 instructs computers wanting to receive IP multicasts that they
> have to add an extension to their Ethernet modules to filter these new
> MAC addresses. It also discusses other Link Layers.
>
> Bert
>


Yea - I wasn't really trying to say anything - just making it clear that
the dst mac was not the mac of my PC because I know that and others
reading may not have known that/how multicast on eth works.

Andy.