Modem MACs and how ISPs view them

MAC address only exists in Ethernet networks or something that emulates it.
Ethernet (Level 2) networks are connected with Level 3 devices called routers.
MAC address is only visible inside of one Ethernet network, so anyone on the other side of the router doesn’t know your MAC address.

Both DOCSIS (cable) modem and DSL modem can run in one of two modes:

  • Router mode. In this mode it acts as a router (Level 3 device) and your Ethernet traffic (Level 2) never goes to the ISP.
    Your PC mac address is not important in this scenario.
  • Bridge mode: modem forwards your Ethernet traffic to the ISP and your MAC address may be important here (at least, your ISP may see it)

With DSL you have a modem (either standalone or built into a DSL router)
Your modem connects to your ISP’s device (called DSLAM).

There is a protocol called ATM that runs on top of DSL.

ATM uses VPI and VCI to create a “virtual channel” between your modem and ISP device.
These two numbers (VPI/VCI) are used to “address” a device inside of ISP’s ATM network.

Then, ATM creates a so-called AAL (ATM adaptation layer) to run some protocol on top of it.

  • It can run IP directly on top of an ATM. There is no MAC address here and only router mode is supported.
  • It can run PPP over ATM (PPPoA): again, no MAC address, much like dialup. PPP runs some protocol to authenticate you (PAP, CHAP or some other). Only router mode is supported.
  • It can emulate Ethernet. It is called LANE. It uses a MAC address! In bridge mode your MAC address is important. In router mode it is not.
  • It can run PPP over emulated Ethernet (previous case but with PPP on top of it). PPPoE. In bridge mode you run PPP on your PC and MAC address may be important
    (sometimes it is not not, if your ISP uses PAP or CHAP to authenticate you). In router mode PPP runs on your modem and PC MAC address is irrelevant.

Most ISPs in my country prefer to run PPP (ether over ATM (PPPoA) or over emulated Ethernet using PPP over Ethernet (PPPoE))
because PPP is a convenient protocol for AAA (accounting authorization authentication)

I’ve seen an ISP that uses PPTP (vpn) over IP over emulated Ethernet. Modem in router mode provides your PC access to the network, and then you run VPN client on your PC.
Again, ISP doesn’t see your PC mac address.

Honestly, I have never seen an ADSL ISP that cares about the MAC address. Usually they are happy with PPP login/password.

With DOCSIS (cable modem) your modem connects to the ISP device named CMTS.
It then downloads configuration from ISP.
There is no MAC address at this moment.

Then, it switches to the one of the two following modes:

  • Bridge mode: modem acts as bridge (ethernet switch). It forwards your Ethernet
    frames from your PC, so ISP is aware of your PC mac address.
    In most cases it happens when you connect your PC to the Ethernet (RJ45) port on your cable modem.

Almost always the MAC address is important in this case! You must tell your ISP which MAC you are going to use.
Or you can change your NIC’s mac address to the one ISP already knows about.

  • Router mode: modem acts as router. It uses its internal MAC address (written
    on small white paper on the modem) and then forwards IP packets to your PC.
    It happens when you connect your PC to the USB port of your modem.

Most DOCSIS ISPs in my country prefer bridge mode and ask for your MAC address.

But if you connect WiFi router to your modem (so you have two different devices) ISP only sees your router’s mac address, not your PC address.
Remember: mac address never crosses your router because router terminates level 2 (Ethernet) network.
https://www.cisco.com/en/US/docs/cable/modems/ubr924/configuration/guide/scg924bc.html

Conclusion

For bridge mode MAC address may be important (but with ADSL chances are high that ISP uses PPP authentication (PAP, CHAP) and doesn’t care about MAC address).
For router mode your MAC address is never important

If you are using a Wifi router then your PC MAC is never important.

Typical setup in my country:
For DSL: Wifi router with DSL modem built into it. It runs some kind of PPP and doesn’t care about your MAC address.
For DOCSIS: Cable modem in bridge mode + Wifi router. You must set your Wifi router MAC address to the one ISP knows about. But your PC MAC is not important.

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