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What would happen if two computers have exactly the same mac and ip ?

We were having a discussion about what would happen if, on a WIRELESS network.

If both have the same IP and mac, considering wireless broadcasts signals everywhere, would both people be able to use the internet or not ?

We were considering this, because both would be able to send and both would be able to receive . I said there would be problems with collisions from tcp connections, but he said that that would not happen.

What would happen?

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closed as not a real question by John Gardeniers, RolandoMySQLDBA, Alex, mdpc, EEAA Jan 28 '13 at 3:20


Think about this: how would your switchgear know where to forward reply packets? –  EEAA Feb 16 '12 at 15:32
If you're using any level of encryption, the session keys for each client wouldn't match the other. Not sure if the second client would even be able to associate because the AP would think there's already an encrypted session between the two, though perhaps it would just disconnect the existing session. If there's no encryption at all, the to clients should operate, though as you said with TCP and related problems. –  Chris S Feb 16 '12 at 15:41
@ErikA if you don't use any form of encryption he just would send it to the IP and MAC the packets came from, thus sending it over the ethernet, which is a wireless signal, which both wireless cards would accept since it's for their mac and ip. –  Lucas Kauffman Feb 17 '12 at 8:52
That works only if you have a single access point. –  EEAA Feb 17 '12 at 13:31
OW that's what I meant, just with 1 ap :) –  Lucas Kauffman Feb 17 '12 at 14:03

2 Answers 2

up vote 1 down vote accepted

Both clients would have IP connectivity but TCP would indeed break (in some cases), effectively preventing web browsing from both clients...

Scenario: wireless client-A and client-B share the same MAC, IP and AP. No encryption is in use.

Client-A initiates a TCP connection to a remote host (e.g. www.facebook.com), but when the TCP/IP stack on client-B hears an unsolicited connection it will send a RST (reset) packet to the remote host, thus tearing down the connection for client-A.

However, if a client has a 'statefull' firewall it will drop the unsolicited TCP packets and therefore the client's TCP/IP stack will never send the RST packet. Thus, if client-A has a firewall then client-B will have working TCP; if client B has a firewall then client-A will have working TCP.

So if both clients are firewalled then neither client may notice a problem.

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From experience I can tell you that this is exactly what happens. –  Michael Hampton Jan 27 '13 at 21:14

There would be a conflict. How does the second device have the same MAC? Through a clone of the MAC? The controlling system should identify the conflict and one system would likley not function. Packets would not be able to properly be forwarded

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If you spoof the mac and ip of someone else on the network. –  Lucas Kauffman Feb 17 '12 at 8:54
