ISP Column - September 2024

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2024-10-17 15:30:04

If you look through the IANA’s IPv4 address registry you will find a set of reservations which collectively are encompasses by the address prefix 240/4, and are annotated in the registry for “Future Use.” These entries reference RFC 1112 section 4, which states: “Class E IP addresses, i.e., those with "1111" as their high-order four bits, are reserved for future addressing modes.” This address prefix encompasses some 268,435,455 IPv4 addresses. From time it has prompted the obvious question: “If we have run out of available IPv4 addresses, then why are some quarter of a billion IPv4 addresses still sitting idle in an IANA registry waiting for an undefined Future Use?” Surely, if there was to be some “future addressing mode” to be defined in IPv4, then we would’ve done it by now. Why can we just add this pool of IP addresses into the all-but fully depleted pool of available IPv4 unicast addresses and relieve, to some small extent, some of the pressures that we have been experiencing with IPv4 depletion over the past decade?

The major points of discussion on this topic were recorded in a couple of Internet drafts from 2008. One of these, draft-wilson-class-e, advocated the redesignation of this address block for private use, extending the set of such local addresses to “assist in the IPv6 transition of larger networks who are using IPv4 in the context of a dual stack deployment.” In such contexts it was reported that the reuse of network 10/8 was not an option because of existing use and potential address clashes [1918bis]. The use of 240/4 offered a more conventional method to connect Consumer Premises Equipment (CPE) Network Address Translators (NATs) to the network’s border Carrier NATs without having to use more involved solutions such as Dual-Stack Lite (RFC 6333), NAT464 or 464XLAT (RFC 6877). Another reason why a private use context was advocated for this address prefix was that it was believed that many IP implementations had implemented this reservation of the 240/4 address block within the IP code itself within end hosts, discarding the processing of any IP packet that had an address from this prefix as either the source or destination address. The address prefix was unsuitable for general use while significant populations of host protocol stacks contained this discard code. The other draft, fuller-240space, advocated the reclassification of this address block as conventional unicast address space, noting that “given the current consumption rate, it is clear that the block should not be left unused.”