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The other day I ran across a piece of software configured to "multicast" using the address 255.5.6.7.

While I've seen local broadcast (255.255.255.255) and "true" multicast (as defined in rfc5771), this address seems to reside outside either of those recommendations. The address did seem to work, however, as a corresponding client was receiving the messages (it was on the same subnet).

I was wondering if this address worked merely since the first octet identified as broadcast, and network devices are coded to shortcut the logic. Does anyone know if this is the case or whether this address falls within another commonly recommended block?

For reference, I already checked RFC919 and RFC5771 and neither seemed to include this block.

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  • That's "class e" space (240/4). It is not multicast, nor is it valid unicast. Many systems will refuse to use it.
    – Ricky
    Jan 26, 2015 at 5:32

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It shouldn't work.

https://www.rfc-editor.org/rfc/rfc1112#section-4

I'm guessing the vendor/developer was assuming it would be used as extended multicast space, but the last proposal for the space was for unicast use.

https://datatracker.ietf.org/doc/html/draft-fuller-240space-02

Are you actually seeing it routed?

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  • It was never approved for unicast usage. Many systems refuse to allow it, and others mistake it for multicast (those systems are broken) For an additional IPv4 /4, it was "not worth the effort"
    – Ricky
    Jan 26, 2015 at 5:34

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