For your question, the ammount of addresses is not the ammount of hosts available.
256 are the addresses, 254 are the hosts as you said be very aware of the terms they use (this can really kill ya

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For 223.255.255.4 being an invalid address, seems totaly nonsens to mee.
As you have read in the RFC the block was reserved BEFORE (so. in the past) It will be available for use.. so this is just a matter of age of your exam.
I think that is a very difficult thing to keep in mind what exam you are taking and it is more then ridiculous that you have to keep in mind what rules applies on your question because of the time the question is asked

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The exams you are taking to prepare are most likely things like Testkings or other preparation, do take note that these exams are not the most up-to-date questions. I hope Cisco will update there questions, but on this one I can't guarantee you how this will be.
Just read your book, see what they say.. and don't 'think' on your exam... thats fatal

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Hope you'll pass very soon m8, Goodluck.