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All the 3 major ISPs in my area require that you specify the MAC address of your external LAN interface. It is only possible to connect to Internet using this MAC address (spoofing also usually works).

What is this limitation for? I think it's to limit the possibilities for vandalism of other users of your local network segment (other clients of this ISP served by the same local switch) towards you, but I can't think of any possible attack.

For instance, without the MAC binding other users could issue IP packets with my IP address as source address, but they have no way to intercept the incoming packets directed to me -> they wouldn't know what to send and where to in order to perform a meaningful attack.

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  • The answer may depend on the ISPs link-layer technologies. DOCSIS cable modems have one reason, and an Ethernet based ISP in an apartment building may have another. What link layers are you talking about here?
    – Spiff
    Nov 1, 2016 at 15:39
  • @Spiff: Ethernet. Nov 1, 2016 at 16:50
  • It stops someone stealing your internet connection by hooking up their own modem ...
    – DavidPostill
    Nov 1, 2016 at 17:33
  • @DavidPostill: stealing how, exactly? First, all the payment plans are unlimited in traffic, and second, there's no need to use MAC as ID because they know which port of the switch every user is hooked up to. Nov 1, 2016 at 18:31
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    Splice into to your line from the exchange and hook in their own modem ... free internet at your expense.
    – DavidPostill
    Nov 1, 2016 at 18:45

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On Ethernet, if they configure their gear to filter out all unregistered MAC addresses, it makes it harder for non-customers to freeload. Also, locking your IP address to your MAC address allows them to avoid ARP poisoning attacks.

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