What happens if there is a device already connected to a wifi network, and subsequently another device tries to join the wifi network using the exact same MAC address as the original?

  • 1
    Is this an actual problem you are trying to resolve? – CharlieRB Dec 14 '12 at 15:43
  • Thanks Charlie, no I'm just wondering how the access point handles it. – Fidel Dec 14 '12 at 15:46

A MAC address is supposed to be unique. If someone sabotages that then two devices on the network will claim to be the same unique NIC. That simply does not work.

Just how it breaks depends on the card and possibly on the driver.

Imagine two people in the same building. Both claiming to be employee number 1.
The left employee #1 shouts 'may I have some coffee?'
Someone fetches coffee and shouts 'Coffee for employee #1 is ready'.
That person now gets two replies, one 'thanks!' and 'Hey, I did not order coffee'.

And just which of the Employee #1's answers first might depend on how busy that person is, adding some more chaos to it.

So briefly: things break.

  • Thanks Hennes, I like your explanation. But in the wifi world, doesn't the AP just broadcast: "Hey employee 1 - here is your coffee" and both the devices receive that message? ie. The air is a shared medium whereas a switch would send it to a single device? – Fidel Dec 14 '12 at 15:51
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    Both receive it. And both should acknowledge receipt. If not a TCP timeout occurs then the AP will broadcast it again. Which means duplicate ACKs received at the AP. (That is assuming the not asking laptop/emp#1 sends an ACK and not a NAK). And I completely ignored possible problems with signing on with WPA2. Thus things can break in even more ways. – Hennes Dec 14 '12 at 15:57

There isn't one answer and this depends on the switches that you use in your infrastructure.

In most cases, both computers will connect to the network and think they are working fine... however, you will see random failures and most likely not see the reason why.

In most Cisco switches, they rapidly flush the old MAC address the moment there is a packet from a different MAC and as such you may be able to do a few activities, but, the majority of network operations will fail.

In most other switches, they remember MAC addresses for around 3 minutes and you will see periods of 0 connectivity.

In all cases - it is highly likely that you will not be aware that the problem is that there is a MAC conflict.

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