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Recently I was looking over RFC6890 (link here) to find out more about the current IP addressing schemes that are in place and noticed a rather large IP address range that is listed as "Reserved For Future Use".

The IP address range in question is 240.0.0.0/4.

Since IPv4 is currently running out of available address (hence the transition to IPv6) why wouldn't the IETF open the 240.0.0.0/4 range to the public so that it could be used as public IP address space? I know that it won't solve the IPv4 limitations and I know that one day we will eventually run out however opening it to the public would give an additional 268,435,455 IP addresses which in my mind is a rather substantial amount that as of the moment is just sitting there doing nothing.

Is there any particular reason that it isn't being used for public allocation?

A secondary question that I have is why is the loopback IP address range (127.0.0.0/8) so large? As of right now (and I very much doubt it will ever change), the loopback IP address range is 127.0.0.0/8 which is 16,777,216 IP addresses. What could we possibly need 16,777,216 loopback addresses for? If you have an answer for this could you please let me know?

Thank you.

Bradley Morrissey
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    It is an inheritance and a combination of _“It seemed a good idea at the time”_ and _“too late to change now”_ – Bob Nov 27 '20 at 23:22
  • All compliant IPv4 devices are required to recognize that range as invalid addressing ("_requires all compliant IP implementations to behave in a special way when processing packets either to or from addresses contained by the address block_"), so it would require changing every IPv4 device, and that simply is not possible. In any case, there are not nearly enough addresses in that range to make even a small dent toward restoring the IP end-to-end paradigm, so IPv6 is still necessary. – Ron Maupin Nov 28 '20 at 00:10
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    [This answer](https://networkengineering.stackexchange.com/a/71130/8499) discusses the size of the loopback block. Basically, at the time of allocation, addresses could only be allocated in classful blocks. – Ron Maupin Nov 28 '20 at 00:17
  • [This answer](https://networkengineering.stackexchange.com/a/53937/8499) discusses the IPv4 address shortage, including the blocks that are unusable, and how that affects the situation. – Ron Maupin Dec 03 '20 at 00:36

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We need an addressing system for the next 5 billion people and 10 billion devices. 0.3 billion IP addresses, even if it were reasonable to salvage them (it isn't), is already not enough.

And really these should be counted in ranges, not individual IP addresses. A couple hundred thousand /22s isn't nearly enough to give every small business provider independent space.

240.0.0.0/4 and 127.0.0.0/8 are intended to be not routable nor globally reachable. Convincing billions of IP devices globally that this has changed is an enormous, impossible effort that will break things. As a small example of this exercise, try in a test lab to carve up 127.0.0.0/8 space, advertise it to hosts, and attempt to get it forwarded.

Why not implement IPv6, if touching every IP node anyway? /64s for every subnet puts many times the size of the entire IPv4 address space into each LAN. Makes counting addresses obsolete. Unused address space, so no conflicts to sort out.

John Mahowald
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