Currently I am in the process of learning the whole DNS thing. I'm learning from the O'Reilly book "DNS and BIND".
In section 3.4.2 the author states that in order for a domain name registrar to be able to delegate a subdomain to a nameserver, that nameserver must be on a registered internet-connected IP network (registered by a network registry). And also, he states the need to double check whether the ISP network, on which the nameserver resides is registered.
The question is: Is it possible for the ISP's network to not be registered by an RIR (or any local network regisery for that matter), because it doesn't make sense to me. I mean, I think that any "Internet-connected" network must be registered by the authority that delegated the IP range.