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I have a few questions, but all under the same umbrella topic:

  1. If I find out a MAC address of a machine on a LAN, and I spoof my MAC to match that particular machine and join their LAN, how will things in the network behave?

  2. I read something that lead me to believe IPv6 has MAC addresses "encoded" in them. Am I wrong in this interpretation? Would it be very bad if someone knew my IPv6 address - could this give them my MAC address also?

  3. Can I get a legal programmable NIC that lets me change its MAC?

  4. Do phones have an equivalent to MAC addresses? I heard it was UUID, but I wasn't sure.

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  • These are all basic networking questions and not security questions.
    – schroeder
    Commented Apr 20, 2016 at 20:27
  • Why do you want a programmable NIC? You can simply change your MAC in software. And what do you mean by "legal"?
    – schroeder
    Commented Apr 20, 2016 at 20:29
  • IPv6 and MAC addresses: en.wikipedia.org/wiki/IPv6_address
    – schroeder
    Commented Apr 20, 2016 at 20:31
  • When you ask about phones, do you mean a MAC address on the cell network or on the wifi network? Phones have a MAC for the wifi network.
    – schroeder
    Commented Apr 20, 2016 at 20:31
  • @schroeder I thought you could only spoof your MAC since it was burned into the netword interface card. Also, I thought phones had something else besides MAC. Do they have MAC for cell network as well?
    – user108251
    Commented Apr 20, 2016 at 20:36

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  1. Your first question is answered in this answer, this answer, this answer, etc. You should search this site to see if your question has been previously answered.
  2. If you use SLAAC to assign IPv6 addresses, the 48-bit MAC address is converted to a modified 64-bit EUI. This has been of concern, and most OSes, by default, use privacy extensions/random address generation. See this answer about this question.
  3. You can change the MAC address which is used with most NICs, but I don't know of any which can be changed in the hardware. You are supposed to set the U/L bit when using your own MAC address, but I really don't see that being followed. I am not aware of there being anything illegal about changing your MAC address.
  4. VoIP phones that run on LANs which use MAC addresses (e.g. ethernet or Wi-Fi) will have MAC addresses.

MAC addresses are only significant on a LAN, and just about anyone on a LAN can get your MAC address from the host ARP cache after communicating with your host at layer-3. To send a layer-3 packet to your host, a host will need to use something like ARP (IPv4) or ND (IPv6) to get your layer-2 address (MAC). Your host will also use ARP (a layer-2 broadcast) to discover the MAC addresses of other hosts, and it announces your MAC address to other hosts.

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  • Most NICs get their MAC from an eeprom (internal or external), or from the BIOS.
    – Ricky
    Commented Apr 21, 2016 at 1:27

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