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Had a conversation with a co-worker that left me scratching my head. Searching the Intrawebz was fruitless in finding an answer. Anyway my coworker was having some speed issues and was working with a level 2 tech over the phone on the fix. He uses cable Internet.

While helping the tech, the tech mention that he could see the internal MAC addresses of my coworkers network externally. As the tech read them out, my coworker was able to verify that it was his and his roommates equipment. How is this possible?

It's my understand that MAC's are only visible if you are connected to the same subnet. Once a packet leaves the network the packet will obtain the MAC of the routers WAN port.

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Ever play the game "telephone" as a kid? My guess is that there is a bit of misinformation mixed in with the real information.

The two most likely solutions I see that could answer this question are as follows:

  1. Cable modem/router gateway device is provided by the ISP and has remote management capabilities the tech was using to view the devices on the network.
  2. Cable modem is actually functioning as a bridge and not a router. Without installing a gateway/router device, the devices are indeed showing up external to the end user network.
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  • Thanks!. I did think there might have been some SNMP magic going on but didnt think of bridge mode. I'm sure there is a bunch of detail missing.
    – nizbit
    Aug 31 '15 at 18:38

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