Like the title says, what happens if my ISP gives my connection (by accident/on purpose) a public IP address that someone else is currently using to browse the web or send an email or pay his bills?

More importantly, what are possible exploits (for me, the other guy with the same IP, a third party maybe) and how are they prevented?

Also, is there any difference with IPv6 vs IPv4 related to this question?

Of course I'm just asking out of curiosity :)


The primary problem in this case is not a security one first and foremost; this situation creates apparent packet loss, as the link layer protocol mapping for your IP address will typically change between you and the other node according to the action of a race condition and much traffic will not go to the requestor. Of course, this does all depend on the link layer.

It's certainly possible that you would see some of each other's traffic, and if you were malicious you could certainly dump it. Of course, you would probably not get all of it. However, this is no different from viewing traffic in transit through a mirror port or by getting it out of a router or something - it wouldn't make it any easier to break the security of SSL sessions, for instance. So, even if this happened (and it would indicate severe incompetence on the part of the ISP), I would not expect an actual security breach unless I wasn't encrypting things.

You'd also see the return traffic of the other person, but not what they send.

In practice, when there are duplicate IPs like this on an ethernet and the nodes do not detect and handle the failure, it's basically impossible to complete the TCP 3-way handshake (let alone negotiate a TLS session) and the connection looks to be extremely poor or entirely down.

Most differences would be as a result of the link layer and how it maps link layer addresses to network addresses, and most security differences would be a function of the application layer protocol being used and whether IPsec is in use. In IPv6, since you are assigned a prefix and that is what would have to conflict, the consequence would be almost identical except that there isn't likely to be anyone on your network with the same IP as the misdirected packets are destined to.

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That is highly unlikely to happen. But if it does, I guess, it wouldn't matter as long as you two don't visit same websites at the same time - which is also highly unlikely.. As for IPv4 v IPv6.. IPv6 is just starting, and I couldn't provide much information about that :)

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    It would matter even if you don't visit the same websites at the same time. When a packet is routed to that IP, the routers don't care where the packet came from - packets from your website would be treated the same as packets from the other person's website, and both routed to your or their computer. – user253751 May 10 '14 at 1:23
  • -1, because returning IP packets are not routed according to their source (the website) but the destination IP, which in this case still would be ambiguous. Your Answer is wrong. – Marcel Sep 16 '14 at 6:06

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