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Converting multicast address

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by robert.godin

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Does anyone know how to you convert a multicast address of 239.222.222.1 to a mac address? When I do it, I get 01005e5e5e02 but I think it's wrong.

4 replies

by rzan

A multicast address is a class D address. Multicast addresses are not assigned to any one machine, so there cannot be a MAC address assigned to it, therefore no entry in ARP. Multicast packets are like broadcasts, but they target a range of hosts,instead of all hosts on an entire subnet. Don't bother looking for an ARP entry for multicast packets because they don't exist. What you may get though in the ARP table is the MAC address of the ethernet interface on your router, which is injectedinto the ARP table as the source MAC address for the packet. Remember, routers don't forward broadcasts or multicast packets by default, they instead regenerate the packet from the destination intertface and inject their MAC address as the source for the packet. I hope this sheds some light on the subject.

by robert.godin

ZThe application we are using requires a MAC address in order to monitor the traffic coming from a specific multicast address. You can fool it by injecting a mac address for any specific multicast bye converting to hex, and this is were I'm not sure if you have to NAND it or XOR the last 2 digits!

by pinar

The button says "post a reply" but I guess mine is mote of a question. You reached any solutions yet? I'm not saying I understand the nature of your problem completely. Since the response you got before this seemed to make sense to me I'd like tolearn your version of what happened.

by dthompson8

This is what i came up with. I took the internet mac address of 01-00-5E-00-00-00 and added your ip address minus the 5 most significant bits to determine it. That is the way i have always converted this. Let me know if you find something different.

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