Broadcast vs Multicast
Marko Lackovic
What's the difference between limited broadcast (255.255.255.255) and
multicast using 224.0.0.1 (defined as multicast to all systems on the
subnet). Is a host by default in group 224.0.0.1, or it has to be explicitly
added (IGRP uses 224.0.0.1. so I presume it is by default in that group).
Thanks.
Wim Lamotte
The implementation of Multicast is quite different from broadcast. If you
send a broadcast packet to 255.255.255.255, the device driver of the sending
machine translates this into an Ethernet broadcast (assuming that Ethernet
is being used), which uses the MAC address ff:ff:ff:ff:ff:ff. All machines
on the same LAN should listen to this MAC broadcast.
On the other hand, multicast IP addresses are translated into "special" MAC
addresses in the range 01:00:5e:00:00:00 to 01:00:5e:7f:00:00, by putting
the last 23 bits of the (multicast) IP address at the end of this MAC
address. Hence, the "all systems on this subnet" address, 224.0.0.1, is
translated into the MAC address 01:00:5e:00:00:01 (you can easily verify
this by pinging to 224.0.0.1 while running a packet sniffer). Now it depends
on the device drivers of the receiving machines if they listen to this MAC
address by default or not. Multicast-aware machines should anyway..
When joining a specific multicast group, a receiver adds the corresponding
special MAC address to the list of MAC addresses it listens to. Note that
since only 23 bits of the multicast address, there is no one-on-one mapping
between multicast IP address and multicast MAC address, so more filtering
has to be done on higher layers as well...
Both broadcasts are not propagated by routers, which limits them to one LAN.
I hope this helps.
Wim
"Marko Lackovic" <marko.l...@fer.hr> wrote in message
news:9tbrc0$8tm8$1...@as201.hinet.hr...
James Davenport
>The implementation of Multicast is quite different from broadcast. If you
>"Marko Lackovic" <marko.l...@fer.hr> wrote in message
>news:9tbrc0$8tm8$1...@as201.hinet.hr...
>> Hi. I have following question:
>>
>> What's the difference between limited broadcast (255.255.255.255) and
>> multicast using 224.0.0.1 (defined as multicast to all systems on the
>> subnet). Is a host by default in group 224.0.0.1, or it has to be explicitly
>> added (IGRP uses 224.0.0.1. so I presume it is by default in that group).
I wrote (excuse the TeX)
One could ask what the r\^ole of {\tt 224.0.0.1} is --- surely
it duplicates the ``limited broadcast'' address {\tt 255.255.255.255}? In
theory that is true, but in practice {\tt 224.0.0.1} means, not Stevens'
``all systems on this subnet'', but rather ``all multicast-capable systems
on this subnet''. Many systems, e.g. printers, are sold without
multicasting, since there is no need for it, and, as we have seen, it
complicates the Ethernet interface, device driver, and the IP layer.
James Davenport
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James Davenport
Wim Lamotte
on the device drivers of the receiving machines if they listen to this MAC
address by default or not. Multicast-aware machines should anyway.."
Many machines won't listen (and hence answer to) packets sent to 224.0.0.1.
This can easily be verified by pinging to this address and tracing the
packets that are coming back...
James, what's the correct link to your Stevens' notes? I can't seem to find
neiter of the ps or pdf versions...
Wim
"James Davenport" <mas...@bath.ac.uk> wrote in message
news:Gn5ntG.7J...@bath.ac.uk...
Skybuck
<Wim.L...@luc.ac.be> wrote:
Hi wim..
A little question:
Could a sender translate an ip multicast address directly into a
ethernet multicast address ?
if that would be possible than arp would no longer be needed to send
udp multicast traffic ?
--
Skybuck presents free software for windows 95 and up:
Network Info, UDP Speed Test v1.07 and UDP Multicast Test v2.00
http://www.mycgiserver.com/~skybuck
Wim Lamotte
The way it is translated is explained in my original reply.
Since when is ARP used in multicast?
Wim
"Skybuck" <removethis....@hotmail.com> wrote in message
news:3bfbc6ee.4907077@news...