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When a request is made, does the router send its own mac address or the mac address of its user? Does it send the mac address at all?

My specific case is the following: I am getting a 4g sim card to plug in a router, but the provider usually limits the internet sharing/hotspotting (tried to hotspot from a smartphone). I am looking for a way to allow everyone to use this card, that is plugged in the router.

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3 Answers 3

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A Switch

  • Is Layer 2 only (uses MAC addressing to send frames)
  • Will broadcast a frame with an unknown recipient to all ports except the one it was received on, until a reply is received. This is used to build the forwarding table.
  • A switch doesn't change the source-MAC address, it passes them on

A Router

  • Is a layer 3 device (in most cases!) and uses IP addressing to send packets
  • Will take the received layer 2 frame, strip off the ethernet header, deal with the encapsulated IP packet, then build a new ethernet header, with its own MAC address to make up the outgoing frame. As pointed out in the comment below, this is only relevant when ethernet is being used, and some technologies in use today aren't Ethernet based at all...

I think in your case, the 4G SIM/LTE modem will just be another 'upstream', and the SIM provider will see a single MAC (that of the routers interface with the LTE modem).

Good luck!

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  • "Will take the received layer 2 frame, deal with the encapsulated IP packet, then swap in its own MAC address to the outgoing frame" That is not completely accurate and is misleading. The router completely strips off the frame to switch the packet to the new interface, then it builds a new frame from scratch for the protocol on the new interface. Not all interfaces use protocols that have MAC addressing. Only the IEEE protocols have MAC addressing. For example, DSL is often on PPPoA. Neither PPP nor ATM use MAC addressing, so the outgoing frame will not have a MAC address.
    – Ron Maupin ♦
    Sep 9, 2019 at 15:06
  • Good clarification Ron
    – jmbrnt
    Sep 10, 2019 at 16:08
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For a MAC-based network, a router uses its own MAC address as the frame source address. However, not all networks are MAC-based and the ISP might not see any address, depending on the connection. In no case does a router make MAC addresses visible between networks.

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Your service provider will see the MAC and IP address of your router.

What service providers do to detect tethering from network traffic is to inspect the TTL. The TTL of packets sent from devices behind the router will be lower than the one sent from the router itself.

There are other techniques as well, see https://android.stackexchange.com/questions/47819/how-can-phone-companies-detect-tethering-incl-wifi-hotspot

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