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We've been provided with a new firmware for our telematics units, which connects to the Internet via a VPN provided by a mobile carrier; since this new firmware release, we've noticed that each and every device (based on Linux) has the same MAC address: DE:AD:BE:EF:00:00

I'm familiar with the workings of Ethernet and WiFi, but still somewhat new to mobile (GSM, LTE, 5G, ...) side of things.

Unless I'm completely mistaken, TCP/IP traffic is still routed to the actual endpoint via MAC addressing on the network layer, hence the need for unique MAC addresses.

Does this also apply to mobile (cellular) networks? If that's the case, that would imply that hundreds of devices (regardless of their IP addresses) with the same MAC address would cause major issues in the field when it comes to connectivity. Or is this a normal practice in the mobile/cellular world?

To date we haven't seen any issues, but we've only been experimenting with this new firmware for a few days and only today have I been able to establish a mobile network connection.

For reference, here's the output of ifconfig

wwan0     Link encap:Ethernet  HWaddr DE:AD:BE:EF:00:00
          inet addr:<snip>  Bcast:<snip>  Mask:255.255.255.248
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:77 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1482 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:3088 (3.0 KiB)  TX bytes:134897 (131.7 KiB)
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    AFAIK mobile networks use completely different layer 1/2 especially on the wireless "part" of the network. Also, these protocols differ betwen network generations. GSM is actually cirquit switched network, so such thing as MAC address should not have a meaning. UMTS should be hybrid (separate procedure calls and Internet), LTE should be packet switched. From my 10 mins googling it seems that wireless layer 2 is not exaclty packet switched. At least I don't see any addresses, except channel names in headers.
    – Effie
    Jan 18 at 16:21
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    here is a list of all protocol names: devopedia.org/5g-nr-mac-pdu
    – Effie
    Jan 18 at 16:22
  • You may note that the MAC address reads "dead beef" which is jargon for unused stuff. It is written using HEX letters only. See more at stackoverflow.com/questions/2907262/what-does-dead-beef-mean
    – Mita Lapo
    Apr 20 at 13:24
  • @MitaLapo I'm aware of Deadbeef and hexspeak. The question has been answered and is considered closed
    – SimonC
    Apr 21 at 8:18

2 Answers 2

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MAC address: DE:AD:BE:EF:00:00

That is a locally administered MAC address (LAA) as indicated by the second-least significant bit in the first octet (0xE = 1110b). As such, it is the responsibility of the user/administrator to make sure it's unique in their network.

AFAIK there's no regulation prohibiting such a practice (there should be) but it may be considered unprofessional. Devices being shipped with identical MAC addresses, without a mechanism to ensure uniqueness (in a network) before use, can be considered broken - but only if these addresses are actually used on an external interface.

TCP/IP traffic is still routed to the actual endpoint via MAC addressing on the network layer, hence the need for unique MAC addresses.

IP uses IP addresses for routing. MAC addresses are used over an unlying, MAC-based data link layer network to enable proper delivery of the encapsulating network frame, but many data link layer networks have other mechanisms for local delivery and don't use MAC addresses.

What you're seeing is likely just a virtual (or abstracted) adapter's dummy address that is presented towards the OS but not actually used anywhere. Mobile devices don't use MAC addresses towards their cell service, they use IMEI addresses. So, MAC duplication problems don't apply here.

('Universal' MAC addresses (UAA) aren't really globally unique any more, but it's the vendor's responsibility to ensure that it's impossible or at least extremely unlikely that two devices with the same address see each other in any network.)

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  • So this would only be the case if the units were to communicate with each other through the VPN, if I understand correctly.
    – SimonC
    Jan 18 at 16:16
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    As it seems, the dummy address is used on the high-level connected-session virtual interface (wwan0 presented to the OS), somewhat comparable to the PPPoE interface used with ADSL. That MAC address isn't actually used anywhere. VPN sits on top of the layer-3 connection, so the L2 interface's MAC address is irrelevant there, see Ron's answer.
    – Zac67
    Jan 18 at 16:31
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Unless I'm completely mistaken, TCP/IP traffic is still routed to the actual endpoint via MAC addressing on the network layer, hence the need for unique MAC addresses.

No, you are mistaken. MAC addressing is used by some data-link protocols to deliver layer-2 frames in the same network. MAC addressing is only ever used and seen on the same layer-2 network. IP has no idea about MAC addressing, and it can be carried by any number of data-link protocols, and not all data-link protocols use MAC addressing.

When an IP packet is routed, the data-link frame is stripped off, losing any layer-2 information, including any MAC addresses. You can certainly have the same MAC addresses on different IP networks because that addressing is not routed between networks.

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  • Thanks for the clarification!
    – SimonC
    Jan 18 at 16:14

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