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Computer A is connected to the internet using router RA that uses home NAT network. In the same way Computer B is connected to the internet using router RB that uses home NAT network.

Lets look at a situation in which computer A sends a massege to computer B through the internet. Let's denote the packet that is sent from A to RA by PA ans denote the packet sent fom RB to B.

Is it neccesery that the mac source in PA be diffrent then the mac destination in PB?

I think the answer is no and my thought is like this. Computer A sends a packet to itself using the IP of the router RA and the port corresponding to computer A under the NAT. Computer A doesn't know he is under a NAT so he wouldn't know that the packet is intended for him. He will send it to the router, the router will send it back to him. And all in all we got a situation in which the mac source is the same as the mac destination.

Am I right? Is it at least a sensible way of thinking about the question? It's from a test I had(multiple choice so they didn't see the argument I thought of) and I didn't get the points, so before writing to them I wanted to consult.

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It's not necessary if they are on different local networks, as the MAC address is only used with a local network. However, odds are that the MAC addresses probably will be distinct anyway in practice, as manufacturers try to keep all MAC addresses globally unique/distinct, to the extent practical.

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  • $\begingroup$ Can you say though if the argument I have givenn above is possible? In the course we assume that the mac addresses are diffrent gllobaly. Many thanks! $\endgroup$ Commented Oct 10, 2024 at 19:21
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Yes, two devices may have the same MAC address, but it's not ideal. The MAC address is meant to be a unique identifier for network interfaces. They are often used in routing enties in layer-2 devices such as switches. When two devices on the same network have the same MAC address, it can cause network conflicts, leading to issues like dropped connections or routing problems.

In most cases, manufacturers ensure that MAC addresses are unique, but they can be manually set or spoofed. In practice, it’s best to ensure that every device on a network has a unique MAC address.

Personal experience: In many cases, at least in India, ISPs track/register the MAC of the router/PON devices and use that as one of the authenticators of legitimate users. I once mistakenly assigned the MAC of one PON device to another and turned both them on simultaneously. The result was, as expected, conflicting routing entires on the ISP's switches leading to several packet drops.

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  • $\begingroup$ Hi thanks for the answer! What I ask is if the situation I presented(when the computer sends a massage to the hole in the NAT) would work $\endgroup$ Commented Oct 14, 2024 at 12:45

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