I knew that any mac address that started with 01-00-5E was a multicast address, but wasn’t sure how the rest of it was calculated. More specifically, being able to tell the hardware address of a device from its IP address is a convenience for administrators, but this same information is available to everyone else as well.Recently I was working to track down a multicast problem for a client on an enterprise network and needed to determine what the exact multicast MAC address would be. This represents a form of information sharing that can be problematic in our modern world of hackers and trackers. However, it violates one of the tenets of layer design, which is that an address in one layer (the hardware address) should not be tied to the addresses at another (the IP address). Linking hardware device addresses to IP addresses offers practical benefits, as explained above. One important facet of this is that “detail hiding” using layers also enhances security and privacy. One of the fundamentals of network design is the use of layers, which allow different technologies to interoperate while hiding unnecessary details that would cause complications in the implementation of each technology. To understand the issue here, it’s necessary to first have a small digression into network design philosophy. The bigger concerns with EUI-64 are those old bugbears that administrators know too well: privacy and security. In practice, this is a relatively minor consideration most of the time because hardware doesn’t change that often-this is something that typically occurs in the scope of years, not hours or days. One disadvantage of EUI-64 is that tying the hardware address to the IP address means that if the hardware address changes, the IP address needs to change as well. Using EUI-64 makes this simpler because seeing the IP address of a device indicates immediately what the MAC address is and vice versa.
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