Re: why multicasting is working?

[Oleg Butorin]

Mike Mestnik wrote:
I'm not an expert on MC, but I'd think 224.0.0.1 would be routed to
     

your
   

default route.  Then the pkt would get multicasted and you would receve
multiple responces.
     

Yes, but I received responces from the systems where multicasting 
disabled in the kernel.
   

There is no difference on the end points, only the routing.  In MC the
routers must(MAY) take one packet and pass on multiple packets.  If a
 

No, the great feature of multicasting is to send only ONE packet to 
multicast address and
other computers, who "subscribed" to this address, will receive this 
packet!
For example (only in theory, I didn't set up this), I have 3 computers 
in my LAN: two workstations
and one router. Multicasting disabled on all computers.
1. I send packet to address 224.0.0.254
2. This is multicast address, that's why on the network interface layer 
packet will have special destination MAC
3. All hosts will receive this packet. There is no work for router in 
this case. Multicast packet will be delivered like broadcast.
4. In this stage I have a question. If system has multicasting enabled, 
it will unpack the packet with the special MAC and
then will decide to answer to this packet or deliver it to the 
application. But I have multicasting disabled, and don't understand
why kernel receive the packet with the special MAC and multicast IP.
5. If router has option to route multicast packets, then the packet can 
be routed to the another network or multicast enabled router.

normal router gets a MC packet it's likely just going to pass it to the
default route.
 

Normal router does nothing with the multicast packets.
IIRC kernel level MC support is only for if you want to be on Mbone,
     

not
   

if you want to use it as a client/server.
     

But the option called "IP: multicasting" and help:
This is code for addressing several networked computers at once,
enlarging your kernel by about 2 KB...
   

It also says if you wish to participate in Mbone.  The code dose allow for
your computer to send more then one MC packet, but a normal kernel will
still send pakets to ANY IP 0.0.0.0 - 255.255.255.255.
 

Sorry, I don't understand, what do you mean "to send more then one MC 
packet"?
All kernels can send packets to anywhere, the question is:
why it receive the alien packet???

And Mbone is the standard network, that supports multicasting (routers, 
computers...).
As I understand, there is no special support for Mbone, this is support 
for Multicasting.

   

That's correct, but without MC enabeled routers your one packet will end
up going to a router with no default route and then you will get a
netunreach ICMP.
 

I don't speak about option "IP: multicast routing". It is clear, that 
this option for routing!

Best regards,
Oleg.
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