IANA gives benchmarks on this topic. For IPv4, here is what they have of their "Distinctive objective address registry". This means, for our functions, just about anything in these address blocks shouldn't be assigned given that they shouldn't be routable "publicly". Stack Exchange community includes 183 Q&A communities like Stack https://gastonh405hbu4.blogmazing.com/35699441/the-basic-principles-of-255-payday-loan