From: Fred Ingham (fningham@xxxxxxxxxxxxxxxx)
Date: Fri Jan 05 2001 - 15:50:13 GMT-3
An Ethernet MAC address with an odd number in the second nibble is a
multicast or broadcast address. For example x1, x3, x5, x7, x9, xb,
xd,
and xf are the possible first byte combinations for an Ethernet
multicast
address.
Some common Ethernet multicasts are:
NetBIOS 0300.0000.0001
Bridge Group 0180.c200.0000 (for BPDUs - IEEE)
IP Multicast 0100.5exx.xxxx
Expanding a bit: a multicast address has the I/G bit (Individual/Group)
set to 1. This bit is the first bit on the wire for a destination
MAC address. Ethernet sends bits on the wire with the low order
(rightmost)
bit first, so this bit must be a 1 for a multicast address.
For multicast IP addresses 224.0.0.0 -239.255.255.255 the last 23 bits
of the IP address are mapped into the MAC address. As an example the
OSPF
all OSPF Router multicast address 224.0.0.5 will be mapped to an
Ethernet
MAC address of 0100.5e00.0005. 239.255.0.1 maps to 0100.5e7f.0001.
Cheers,
Fred
Lykourgiotis Paraskevas wrote:
>
> Hi,
>
> How can I say if a given Ethernet mac address is a multicast one?
> Furthermore do all unicast mac addresses start with 00-....?
>
> Thanks
>
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