[talk] Whaaaaa? Unicast Use of the Formerly Reserved 127/8

N.J. Thomas njt at ayvali.org
Fri Nov 19 14:33:34 EST 2021

* Isaac (.ike) Levy <ike at blackskyresearch.net> [2021-11-19 14:13:25-0500]:
> "This document provides history and rationale to reduce the size of
> the IPv4 local loopback network ("localnet") from /8 to /16, freeing
> up over 16 million IPv4 addresses for other possible uses."

Years ago, before v4 exhaustion, similar calls were made made to request
organizations like MIT to give up their /8 IP allocations and the
standard response from all the network engineers was that it would only
delay the inevitable for a few months at most.

The IPv4 pool was completely exhausted a few years ago, so even if this
proposal would be accepted (I strongly doubt it would be), it would be a
small band-aid applied to a gushing wound.

(MIT did eventually sell off a large chunk of their /8 a few years ago
for lots of $$$. They probably should have waited, it's worth a lot more

As much as I am annoyed with IPv6 (lack of backwards compatibility being
its biggest wart), it is the way to go; any other attempts such as this
draft, and you're only complicating matters. We now have NAT, CGNAT
(https://en.wikipedia.org/wiki/Carrier-grade_NAT) and the amazing STUN:


to fix the problems we created trying to fix the problem. =-)

As expected, this proposal is being excoriated on NANOG, so I'll leave
it to you fine folks to look up that discussion. I'll just quote the
inimitable John Levine on the subject:

    The amount of work to change every computer in the world running
    TCP/IP and every IP application to treat 240/4 as unicast (or to
    treat some of 127/8) is not significantly less than the work to get
    them to support IPv6. So it would roughly double the work, for a 2%
    increase in the address space, or for 127/8 less than 1%.  The code
    for IPv6 is already written, after all.

    Also, while the world has run out of free IPv4 address space, there
    is plenty of IPv4 if you are willing to pay for it. A 2% increase in
    v4 addresses would not change that.


More information about the talk mailing list