In a current project a number of addresses is needed for closed IP communication between a number of institutions.
That is an internetwork, often called an internet. The Internet Protocol (IP) was invented specifically for this purpose.
The logical choice for this is some RFC1918-space, but certain people fear that no matter what 1918-addresses we may choose they may be in use internally in the involved networks and thus unavailable to this project.
Those people are right. The logical choice for IP addresses on an internet is to use addresses from the RIR. It doesn't matter whether this internet is connected to the Internet or not.
Someone else, however, picked up on the fact that there's a large number of addresses just marked "reserved" by IANA, and they probably are not in use, so why don't we just grab e.g. 82.0.0.0/8?
Because all of the organizations in your internet will also be connected to the Internet as well. When the 82/8 space starts to be used on the Internet, each organization will have to deal with routing and/or reachability issues in their internal networks. In order to avoid this kind of mess, the IETF set up the IANA so that organizations building TCP/IP networks could register for globally unique IP addresses. Even though the mechanisms have changed since then (CIDR and the RIRs) the basic principle still holds. There is no requirement that an organization must allow everyone to access their network in order to justify globally registered IP space. It is better for you to register the number of addresses that you actually need rather than hijack the entire 82/8 range. -- Michael Dillon