What will happen in something like a wifi connection where different laptops are set with the same MAC address . To make it more interesting what will happen if there are multiple devices with same MAC address and different IP addresses.

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    This isn't the right place to ask this question, but here goes. If 2 devices with a different IP had the same MAC then packets destined for one IP address will land at both devices and would likely be ignored by the device with the wrong IP.This might even work to some extent, though communication between both devices would fail. If both had the same IP and mac, the same thing would happen, although there would be no way to address either machine individually in any (even broken) way. – Alex Berry Sep 22 '15 at 12:46

I had this problem some time ago; there were two ethernet cards with the same MAC address in a 10 stations LAN. In auto configuration mode (DHCP), the first computer to boot would take an address and the second was given the same address by the DHCP server. (The switch was stupid enough to never mind the MAC address conflict). Then, I tried giving the second computer a static IP, but still nothing came out of it. Since it was a LAN, the server was "talking" with the client at the MAC address level and the switch would always pick one client to send the data, seemingly randomly. So, there was no chance of a separation at the IP level on the client side, since the switch randomly disconnected a client from the data flow. Finally, I had to hard code a different MAC address to have a functional network again.

My guess is, if I had a HUB instead of a switch, things might had worked out. Both clients would have received the packets sent from the server and the one with the right IP address would have processed the received data. But since HUBs have been obsoleted by switches for about two decades, I wasn't able to test this scenario.

  • Yeah the problem and the solution did occur to me when I was attending the lecture on MAC address. – Chris Thaliyath Sep 22 '15 at 19:32
  • I think it would be wise to communicate purely on IP addresses ie strip the MAC address at the switch level .This would make sure that even the peers in the local network would not get access to the MAC address (unless the mess around with the Kernal of the switch/router ). Well if the MAC address is only shared between the router and the node the. It would offer added security within the LAN – Chris Thaliyath Sep 22 '15 at 19:35
  • It sounds nice, but only with a layer 3 switch. Layer 2 switches know nothing but MAC addresses, so stripping them off at any level would create havoc. Just try to figure out what would happen if you had a pc-switch-switch-pc scenario. The second switch wouldn't have any knowledge of the involved MAC addresses (since they would have been stripped by the first switch) and wouldn't be able to handle the packets. – gmelis Sep 22 '15 at 19:49
  • Yeah now I understand , I think every device should have a dynamic MAC address with in the LAN . This would ensure the anonymity of the user . I mean for every session under a Local Area Network, your device should be assigned a particular MAC address . – Chris Thaliyath Sep 22 '15 at 19:57
  • And guess what, there's a number of utilities that does just that. For Linux and *BSD at least I know there are, but there must be a few for windows and mac systems too. In fact I had to disable this feature in my laptop, because I was getting a different IP address every time I connected and I couldn't get my custom configuration from my DHCP server. – gmelis Sep 22 '15 at 20:06

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