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I've developed a Multicast UDP client/server application, which allows a user to remotely change the network configuration of devices belonging to the same multicast group. But what if a device was assigned the same IP address as another device within the same LAN? Will both devices receive the Multicast UDP request? For instance, consider the following:

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DEV_0 does not belong to the multicast group 226.1.1.1. DEV_1, DEV_2 and PC (Client) do.

So if the client sends a UDP message (request), to group 226.1.1.1, do DEV_1 and DEV_2 both receive that request? Wouldn't duplicate IP addresses mess up the entries in the IGMP table?

NOTE: addressing the devices using my application is done using serial numbers! This way I can distinguish between devices. Of course the devices must first receive the request in order for the whole application to work.

H_squared
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1 Answers1

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No.

Multicast packets are sent to a special address where only the devices in the group are listening at. This is independent of your main (unicast) IP address. The multicast IP address is tied to the devices' MAC address on the network fabric (AKA switch infrastructure) and not to their main IP address. So effectively your multicast traffic won't be affected by an IP address duplication, but any other traffic will.

If you'd like to make the test, configure a preferred IP address that's out of the network's address space (or even better, configure an APIPA). Then capture some traffic and run your multicast application with the device added to your multicast group. You'll see it working.

Pedro Perez
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  • "tied to the MAC address"? Not really, the device captures all multicast traffic to any group it's a member of, MAC addresses are irrelevant here. – jch Feb 15 '15 at 18:01
  • Hey jch, I was under the impression a switch wouldn't flood all its ports with Multicast traffic because that's why you have Broadcast. That's why I said "tied to the MAC address" although I poorly specified how and where. From Cisco: There must be a mechanism that informs the network that the computer is a member of a particular group. Without this information, the network would be forced to flood rather than multicast the transmissions for each group. I've edited my answer. – Pedro Perez Feb 16 '15 at 16:44
  • Pure layer 2 switches flood multicast frames. Some switches implement "IGMP snooping", which is a horrible, cross-layer hack. – jch Feb 16 '15 at 22:35