an aspect i did not notice to be covered in the response found below is the
following:
it appears to me that multicast technology incorporates control of traffic
distribution into the addressing scheme: all
multicast-rfc-compliant-devices will forward traffic to multiple hosts if
the destination address warrants it.
it also appears to me that directed broadcast is dependent upon the
configuration of each device in the potential path of a given wayward ip
packet: if a device is configured to forward directed broadcast, the packet
ambles on. if not, it expires in the fashion of most packets, within the
confines of the circuitry of the device that last received the packet.
so, with a directed broadcast, your device is indiscriminately forwarding
multiple-destination traffic on a per-interface-basis, while a multicast
implementation allows for some means of control as might be implemented
from the source end.
i'd humbly ask that members of the group advise everyone if it is the case
that, as usual, i'm way off base.
Jeff Kell <[EMAIL PROTECTED]>@groupstudy.com on 12/28/2000 01:23:00 AM
Please respond to Jeff Kell <[EMAIL PROTECTED]>
Sent by: [EMAIL PROTECTED]
To: Hunt <[EMAIL PROTECTED]>
cc: [EMAIL PROTECTED] (bcc: Kevin Cullimore)
Subject: Re: Difference between Directed Boradcast and Mulitcast
Hunt wrote:
>
> What is the difference between Directed Boradcast and Mulitcast?
A multicast is an IP address 224.x.x.x/4 (224-239.x.x.x) while a
directed broadcast can be to any class A-B-C subnet; the directed
broadcast having all ones in the host part of the subnet address.
A "directed" broadcast is one which "may" be routed. Similarly,
multicasts "may" be routed, and in the absence of any other parameters
it will be flooded across all router interfaces, while a directed
broadcast goes to a specific interface.
At the layer 2 level, broadcasts use the all ones MAC address of
FF-FF-FF-FF-FF-FF. Multicasts have the second hex digit "odd" so that
the "little-endian" nature of the ethernet results in the low-order bit
of the second hex digit being transmitted first. The NIC hardware
will detect this and signal an interrupt in anticipation of a
broadcast. Smarter NICs will continue to receive and match the source
MAC (if not a broadcast) with designated multicast addresses it has been
told are "interesting".
There is also an interplay between multicast MACs and IPs (left as an
exercise for the reader). For the first three bytes of the MAC, the
vendor ID (in the first three bytes) is preserved but the low order bit
of the first byte is set, making it odd (the second hex digit). For
example, an HP manufactured NIC might have an 08-00-09 prefix; but
HP specific multicasts will have an 09-00-09 prefix.
Jeff Kell <[EMAIL PROTECTED]>
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