[techtalk] MAC addr from Cable Modem

Courtney M Eckhardt cme at MIT.EDU
Thu Jan 20 19:00:09 EST 2000

In message <00012003072500.32468 at c21a185.neo.rr.com>, "Wendt,Andrew" writes:
>On Wed, 19 Jan 2000, Courtney M Eckhardt wrote:
>>The idea with running tcpdump on the local section of the ISP's
>>network is that you can see the arp requests, which will be (roughly)
>>in the form of "who-is foo tell bar" with foo being an IP address and
>>bar being the MAC address of the requesting party.  Admittedly, this
>>method involves an awful lot of noise, since you only want one small
>>datum... and matching up your IP with your MAC address would probably
>>take until some machine cared about you and made an arp request...so I
>>think the method I would have to reccomend is talking to your ISP, as
>>they will almost certianly know. :) 
>Wouldn't this method show the MAC address of the ethernet card connected to th
>modem, not the modem itself?

Hrm, you may be right... it may also depend on your service.  I
*thought* what we saw was modem MAC addresses, but maybe I'm on crack.

>Your machine will ARP your gateway for outgoing packets not destined for the
>local network instead of the modem. And people who send packets to you are
>going to ARP on your IP, probably not the modem's IP, if it even has one...

Well... at least the way my service works, you get one IP.  I
*thought* that was associated with the MAC address of the modem
(someone else arps the gateway for you, they get the MAC address of
the modem), but perhaps not, or perhaps it's dependant on your service.

techtalk at linuxchix.org   http://www.linuxchix.org

More information about the Techtalk mailing list