Re: Modem MACs and how ISPs view them, was Re: on the verge of …

From	IL Ka <kazakevichilya@gmail.com>
Newsgroups	linux.debian.user
Subject	Re: Modem MACs and how ISPs view them, was Re: on the verge of …
Date	2021-03-10 00:10 +0100
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I will try to share my knowledge about DSL and DOCSIS (cable modems)
although it may be inaccurate and/or outdated and valid only for my country.

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.

https://flylib.com/books/3/206/1/html/2/images/0130938106/graphics/14fig02.gif

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.

Thread

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      Re: on the verge of shopping for new desktop hardware, recommendations? Dan Hitt <dan.hitt@gmail.com> - 2021-03-07 19:10 +0100
  Re: on the verge of shopping for new desktop hardware, recommendations? IL Ka <kazakevichilya@gmail.com> - 2021-03-07 14:20 +0100
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            Re: on the verge of shopping for new desktop hardware, recommendations? Dan Hitt <dan.hitt@gmail.com> - 2021-03-08 01:40 +0100
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