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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