Significance of MAC numbers.

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I know that many hardware devices have MAC numbers. What are the
significances of these numbers. Why are they these particular numbers?


Re: Significance of MAC numbers.

species8350 wrote:
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Probably easier to use Google than to post the question here and certainly

John McGaw
[Knoxville, TN, USA]

Re: Significance of MAC numbers.

species8350 wrote:
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MAC numbers are used to address Ethernet packets.

    ( )

Here, you can see the IP address, is carried separately from
the MAC source and destination addresses. The addresses are at
different layers in the protocol stack.

The MAC address has significance, between point to point devices.
The IP address has network wide significance, as the IP address
can be preserved as a packet is forwarded from one point to the next.

        MAC #1              MAC #2                 MAC#3
        Device ------------ Device --------------- Device
        IP #1                                      IP #2

And, of course, Windows activation places a significant weight
on the value of the MAC address, on the assumption the Ethernet
chip is soldered to the motherboard, and the MAC address can be
used to detect a hardware change :-) MAC addresses are sometimes
used with cable modems, as a user validation mechanism (so change
the device connected to the cable modem, and it stops working -
this can be fixed with MAC address spoofing).

This HFS24Tusermanual2.1.pdf document, on page 8, shows another purpose
for the MAC address. It allows a network switch to more efficiently
forward packets, instead of sending them to all ports on the
switch. Using address learning, the packet is sent to a destination
port, based on storing MAC addresses and learning who has what

As for the particular values, if you're manufacturing Ethernet cards,
you buy MAC addresses in blocks. Some of the digits, are intended to
identify the hardware manufacturer. The purpose of buying MAC address blocks,
and the attendant registration, is to guarantee the numbers are unique.
Using unique values is important for identifying local devices (since
the MAC address doesn't travel across the entire Internet). For example,
that Hawking switch, if two Ethernet cards had the same MAC address, is
going to do a poor job of forwarding packets.

So that is what I've learned so far.


Re: Significance of MAC numbers.

On Tue, 15 Sep 2009 04:55:58 -0700, species8350 wrote:

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Basically, each MAC address was a unique number identifying each piece of
hardware. it identified manufacturer, model and individual devices.

Mac addresses must be unique for each segment.

Some modern hardware allows you to change the MAC address which is useful
if your hardware has a vulnerability. No sense in labelling deficient
hardware for those who might want to take advantage.

In practise, there is hardly any need to change it.

Use; basically, when a packet arrives at the gateway to that segment, the
gateway sends it to the device having the MAC address that it has matched
to the IP address on the packet. If it doesn't have and IP & MAC listing
in the take, it issues an arp command like "who has ip a.b.c.d" and the
box configured with that IP responds.

note, if it was some other protocol (novell, netbeui. ?) than IP it still
responds with the MAC address.

Re: Significance of MAC numbers.

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Thanks to all for the information and the links.

Best wishes.


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