[techtalk] MAC addr from Cable Modem

Wendt,Andrew awendt at neo.rr.com
Thu Jan 20 03:47:34 EST 2000

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 the
modem, not the modem itself?

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...

My tcpdump allows a -e switch which shows the source and destination MAC
address for each packet, not just ARPs. In my case, I get the MAC address of my
gateway instead of the modem though.


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