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I would like a summary of common scenarios that can cause a MAC address to change?

  • Does hopping on a new network give you a new MAC?
  • Does installing a new network device (e.g. WiFi card) cause a change?
  • In particular, how does this work on Amazon Web Services? If you have an instance that you keep spinning on/off, does it retain the same MAC address?
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up vote 6 down vote accepted

It looks like Amazon don't allow you to choose a specific MAC address but instead allocate you one on machine creation.

Based on

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GOod link. Thanks. – Simon Oct 24 '11 at 9:56
To clarify, it appears to allocate a new MAC everytime you re-launch an instance. – Simon Oct 30 '11 at 2:54

Nope, joining to a new network won't give you a new address.
Installing a new device, won't change the existing MAC address on your other card.
It'll, it is a single 'emulated' network device.

The only thing that can more likely to change your address is a driver update which happened to me many times during the past. When I have an older card, and the manufacturer got sold, or someone bought the company ... something like that, THAT part of the MAC address will change when I install a new driver. (The first 3 bytes.)

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In order to have fixed MAC address in AWS EC2, you can use an EC2 instance in a VPC and attach an Elastic Network Interface to it. ENI is like an additional network card that have fixed IP address (internal and external) and fixed MAC address.

More about ENI in AWS site.

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A MAC address is the physical address of the layer two medium, and in general will remain with the hardware it is allocated to - in a physical nic, it resides in the firmware or flash on the nic. MAC addresses can be spoofed such as with macchanger on linux so as usual, nothing is concrete. They can also be altered in flash - in some cases fairly easily. However, as the MAC address is bound to hardware, the MAC address changes with hardware changes, not with network changes.

In the case of virtual hardware, it is largely the same deal, except the allocation of a mac address is up to the host, effectively it "creates" the nic that is given to the guest, and so seeing as none of it is real, the mac is made up, and can be changed at any time. It rarely is however. A mac address remaining static is good for virtual platform providers as they can measure all sorts of things based on mac, and of course, allocating IP addresses.

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No it doesn't. The MAC address of a NIC will never change. Once you are in a network it uses RARP to get its logical address from the network.

Now say you install a new NIC and join a network (say the Internet). Each card will get their unique IPs provided by the ISP.

So NIC-1 will mind its business and would not interfere with NIC-2 and NIC-2 would do the same. This is true for any number of NICs, irrespective of the medium. They all have their own MAC addresses and hence would acquire unique logical addresses.

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Mac address can change when installing a new ethernet card.Also when switching internet providers can change a mac address. Sometimes a mac address changes when install some software and/or drivers.

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ISPs cannot change your device's MAC address. – Heptite Apr 14 '14 at 1:08

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