I found this question, but I'm still wondering if there were a way that a router could forward a laptop's MAC address to the modem, and if the modem could forward it to the ISP. Are there routers and modems that are modified to do this, such as the ones that ISPs give to their customers?

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    What kind of modem? Cable? ADSL? (IPoE or PPPoX?) WISP? Dial-up? – user1686 Aug 2 '19 at 4:55
  • There are apps that can pass the MAC address inside other communications; sometimes used for security purposes on games & such to stop a single player trying to be 6 different characters in a MMORPG etc. Often the data isn't used or even saved unless that person comes under official scrutiny. [I work support for one such structure, but as it's not public knowledge, I cannot say which.] – Tetsujin Aug 2 '19 at 8:11
  • An application running on a machine can determine the MAC address of a network adapter easily enough. Likewise, a router knows the MAC address of the client (hence the reason you are able to create a whitelist), and software running on the modem or router could do something with that information. Knowing the MAC address of the clients connected to a network isn't useful information when Windows 10 can generate random MAC addresses for adapters, or you can simply change the MAC address of the adapter yourself (device properties). Could this be done, sure, if the firmware is written to do it. – Ramhound Aug 2 '19 at 14:28

Modems are bridges and work at the link layer (where Ethernet runs). They can vary a bit depending on the WAN technology they work with. But if the WAN connection deals with Ethernet frames (such as VDSL, GPON, DOCSIS, or ADSL IPoE) then the entire frame, including MAC addresess, is preserved. Whatever MAC address the modem sees, the ISP also sees.

Routers work at the network layer (where IP runs). They do not preserve the lower layer headers – all MAC addresses are stripped on input, and a new Ethernet frame is generated on output. From the modem's and the ISP's point of view, everything is sent from the router's own MAC address.

Routers (like hosts) can be configured to use any MAC address for their interfaces. If your ISP or modem expects a specific source MAC, you can easily configure that on your router's "WAN" interface. This is often called "MAC address cloning" in the router's settings screen.

However, even if you use MAC cloning, a router will still use only one MAC address per interface – it will not dynamically preserve the MAC addresses of all your internal hosts, and I don't think I've seen a configuration that would do so while still preserving the routing function.

The only time ISP-issued devices preserve MAC addresses of every LAN host is when they're configured as bridges, not routers. For example, when you have a multifunction modem, it'll use its own MAC address if routing is enabled, but it'll preserve MAC addreses if bridging is enabled.

  • Is there any way to verify that the modem is indeed only receiving the mac from the router and never any macs from devices connected to the router? How would that be done? – mrbelvedere Aug 3 '19 at 3:01
  • Connect another device between them -- e.g. a computer with two Ethernet interfaces bridged together, or a switch with port mirroring function -- and monitor the frames going out from the router via the bridge into the modem. – user1686 Aug 3 '19 at 11:33

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