From: Paulo Mendes De Oliveira (pmendesd@xxxxxxxxx)
Date: Fri Jan 05 2001 - 14:28:50 GMT-3
L2 multicast addresses on Ethernet always start 01.00.5E. However make
sure you understand the relationship between L2 & L3 multicast
addresses, especially how L3 multicast addresses map onto L2 multicast
addresses.
L2 has an important role to play in the determination of exclusive i.e.
unique multicast addresses. The IANA owns a block of Ethernet addresses
that in hexadecimal is 00.00.5E. This is the high-order 24 bits of the
Ethernet address, meaning that this block includes addresses in the
range 00.00.5E.00.00.00 to 00.00.5E.FF.FF.FF. The IANA has allocated
half of this block for multicast addresses. Given that the first byte of
any Ethernet multicast address must be 01, the Ethernet addresses
corresponding to IP multicasting are thus in the range 01.00.5E.00.00.00
through 01.00.5E.7F.FF.FF.
Hence the first 24 bits of the 48 bit Ethernet address are always
01.00.5E for a L2 multicast address. However L2 multicast addresses also
use the 25th bit of the 48 bit Ethernet address therefore, implicitly, a
L2 multicast address always has the same 25 bits in the high-order 48
bit Ethernet address.
Given that an Ethernet address is 48 bits, this leaves 23 bits (48 bits
25 bits) in the Ethernet address to correspond to the L3 multicast
address. These low-order 23 bits of the Ethernet address are then mapped
to the low-order 23 bits of the L3 multicast address.
This mapping presents a challenge at L3 as follows:
A multicast address is 32 bits
The multicast address range is always represented in binary as 1110.
This is equivalent to the 1st 4 bits,
The next 5 bits are ignored in order to map the L3 multicast address
onto the Ethernet address
This leaves 32 bits less 4 less 5 = 23 bits of the L3 address
These 23 bits (of the L3 address) are then mapped to the low-order 23
bits of the Ethernet address.
Therefore since the upper 9 (4+5) bits of the L3 multicast address are
ignored in this mapping, the resulting L2 address is not necessarily
unique.
Hence there is a 32:1 overlap i.e. 32 L3 multicast addresses can be
represented by 1 L2 multicast address.
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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