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I was speaking with tech support for my cable internet provider troubleshooting a faulty router. They knew, somehow, if and when my modem was communicating with my router. Just wondering how is this possible? Can they detect the NAT somehow (how?)? Thanks.

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2 Answers

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It's possible that your modem reports the MAC address back to your ISP. If that MAC address points to, say, cisco, or broadcom, then it's obvious you're using some form of router, as neither manufacturer commonly makes NICs.

EDIT: They could also be detecting the operating system installed on the router using any form of operating system fingerprinting.

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not sure I follow. my modem itself has a MAC address – jkm Dec 3 '10 at 3:46
@jkm: Any device which is ethernet has a MAC address. Your computer, your modem, and your router. It's a simple matter for your modem to report the mac address of the device to which it is connected. – Billy ONeal Dec 3 '10 at 3:47
so bottom line is that the modem was reporting the mac address of the router ... – jkm Dec 3 '10 at 3:51
@jkm It's possible, yes. – Billy ONeal Dec 3 '10 at 3:55
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You ISP can log into the modem, inside your house. Once there, they can easily see what is connected to it, and what those connections are doing. As suggested elsewhere, they might use OS fingerprinting to recognize that you've plugged a router into the modem, and not a direct connection to a computer.

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Well.. not directly - a NAT/NAPT box effectively presents a single PC to the modem. The modem would not, for example, be able to discover your overall network topology, only the nearest device connected to it. – Billy ONeal Dec 3 '10 at 5:42
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