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Do modems have MAC addresses?

Posted on 1999-11-08
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Do modems have unique MAC addresses? If so, are they unique to LAN cards MAC addresses? If not, what is written in packets sent from modems in the MAC address space?
Thanks.
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Question by:gilbert_chang
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by:TedSenn
ID: 2191534
I believe that the MAC address that is assigned to the modem is assigned by the software. The modem doesn't have a permanent (ROM balsted) MAC address.
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by:Otta
ID: 2193184
Modems use SLIP (Serial Line Interface Protocol) or PPP (Point to Point Protocol), rather than TCP/IP.
So, there may be no need to insert a MAC-address into a SLIP/PPP packet.
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by:gilbert_chang
ID: 2193536
Otta - You are partly right. Modems use SLIP or PPP UNDER TCP/IP, not rather than.
I checked it - because SLIP/PPP are serial connections they do not need a MAC address. They connect only 2 points at any given time anyway.
Please post an answer so I can transfer the points.
Thanks.
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by:j2
ID: 2193786
A slip / PPP connection DOES have a MAC adress *requisite for TCP/IP more or less). The MAC is however (if we are talking Windows here) Generated by the Dial Up networking adapter
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by:gilbert_chang
ID: 2196177
j2 - A SLIP/PPP connection only has the MAC address of the LAN adapter at the ISP (when continuing to travel in the Internet after the serial connection), since only it starts sending packets on Ethernet. Till that LAN adapter, there is no Ethernet, only a serial connection, thus no MAC address is needed.
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by:j2
ID: 2196222
No. Every 'Dial up adapter' in every Win9x system does have a software generated MAC adress.

(Below snipped from a generic online resource)

"The Dial-up Adapter generates a "fake" MAC address (a 48-bit number representing an Ethernet address) so that protocols designed for Ethernet will work on it. It will generate a random number for the MAC address each time you dial in. This happens REGARDLESS of what protocol you dial in with."
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by:j2
ID: 2196239
But to answer the original Question.

'No a modem does NOT have a MAC adress, ethernet devices, and devices carrying ethernet protocolls must have a MAC adress. Therefor the DUN adapter in Win9x generates a MAC adress when you dial in. This is to facilitate, say, IPX/SPX. No two devices should have the same MAC adress, the first part of a MAC adress is an ID of the manufacturer. Then it is up to the manufacturer to make sure all the MAC adresses they assign are unique. This isnt always the case, Compaq managed to ship some 5.000 Deskpro machines in Europe with the same MAC (that was fun sorting out)'
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by:gilbert_chang
ID: 2196460
j2 - when looking at the SLIP/PPP packet structure, there is no place for MAC. Since when dialing in the layers are: PPP/SLIP,IP,TCP I do not understand where the fake MAC address generated is actually put in the layer model.
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by:j2
ID: 2196471
IIRC MAC is transmitted during negotiation, then it is considered static for the remainder of the session.
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by:gilbert_chang
ID: 2196489
Thanks.
Would YOU like to post an answer so I can tranfer the points?
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j2 earned 200 total points
ID: 2196504
isnt there a link 'accept comment as answer'?
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by:gilbert_chang
ID: 2196722
This is a new feature, isn't it?
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by:Otta
ID: 2199089
> Otta ...
> Please post an answer so I can transfer the points. Thanks.

Hmmm. A case of "I snooze, I lose".  
Oh well -- if I did accept the points, I would have to figure out how to use them.  :-)

> Modems use SLIP or PPP *under* TCP/IP, not rather than.

It's the other way around;
SLIP (or PPP) encapsulates the IP packets,
so IP is running "under" the control of SLIP (or PPP).
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by:gilbert_chang
ID: 2199519
Sorry about the points Otta. Accept my appreciation.
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