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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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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
    
Exactly, also they will sometimes see weird stuff like like multiple MAC addresses, or after a reboot one PC that was reporting might be replaced by another, so instead of Intel it comes back up as Broadcom. Seeing a Netgear device MAC is also a dead give-away, as stated earlier with Cisco. –  AthomSfere Apr 8 '13 at 17:36
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