5 min read

Last month, the team behind Linux kernel announced a patch that allows 0.0.0.0/8 as a valid address range. This patch allows for these 16m new IPv4 addresses to appear within a box or on the wire. The aim is to use this 0/8 as a global unicast as this address was never used except the 0.0.0.0.

In a post written by Dave Taht, Director of the Make-Wifi-Fast, and committed by David Stephen Miller, an American software developer working on the Linux kernel mentions that the use of 0.0.0.0/8 has been prohibited since the early internet due to two issues.

First, an interoperability problem with BSD 4.2 in 1984, which was fixed in BSD 4.3 in 1986. “BSD 4.2 has long since been retired”, the post mentions.

The second issue is that addresses of the form 0.x.y.z were initially defined only as a source address in an ICMP datagram, indicating “node number x.y.z on this IPv4 network”, by nodes that know their address on their local network, but do not yet know their network prefix, in RFC0792 (page 19).


The use of 0.x.y.z was later repealed in RFC1122 because the original ICMP-based mechanism for learning the network prefix was unworkable on many networks such as Ethernet. This is because these networks have longer addresses that would not fit into the 24 “node number” bits.

Modern networks use reverse ARP (RFC0903) or BOOTP (RFC0951) or DHCP (RFC2131) to find their full 32-bit address and CIDR netmask (and other parameters such as default gateways).

0.x.y.z has had 16,777,215 addresses in 0.0.0.0/8 space left unused and reserved for future use, since 1989.

The whole discussion of using allowing these IP address and making them available started early this year at the NetDevConf 2019, The Technical Conference on Linux Networking.

The conference took place in Prague, Czech Republic, from March 20th to 22nd, 2019. One of the sessions, “Potential IPv4 Unicast Expansions”, conducted by  Dave Taht, along with John Gilmore, and Paul Wouters explains how IPv4 success story was in carrying unicast packets worldwide.

The speakers say, service sites still need IPv4 addresses for everything, since the majority of Internet client nodes don’t yet have IPv6 addresses. IPv4 addresses now cost 15 to 20 dollars apiece (times the size of your network!) and the price is rising.

In their keynote, they described, the IPv4 address space includes hundreds of millions of addresses reserved for obscure (the ranges 0/8, and 127/16), or obsolete (225/8-231/8) reasons, or for “future use” (240/4 – otherwise known as class E).

They highlighted the fact:

“instead of leaving these IP addresses unused, we have started an effort to make them usable, generally. This work stalled out 10 years ago, because IPv6 was going to be universally deployed by now, and reliance on IPv4 was expected to be much lower than it in fact still is”.

We have been reporting bugs and sending patches to various vendors. For Linux, we have patches accepted in the kernel and patches pending for the distributions, routing daemons, and userland tools. Slowly but surely, we are decontaminating these IP addresses so they can be used in the near future. Many routers already handle many of these addresses, or can easily be configured to do so, and so we are working to expand unicast treatment of these addresses in routers and other OSes”, they further mentioned.

They said they wanted to carry out an “authorized experiment to route some of these addresses globally, monitor their reachability from different parts of the Internet, and talk to ISPs who are not yet treating them as unicast to update their networks”.

Here’s the patch code for 0.0.0.0/8 for Linux:

Users have a mixed reaction to this announcement and assumed that these addresses would be unassigned forever. A few are of the opinion that for most business, IPv6 is an unnecessary headache.

A user explained the difference between the address ranges in a reply to Jeremy Stretch’s (a network engineer) post, “0.0.0.0/8 – Addresses in this block refer to source hosts on “this” network. Address 0.0.0.0/32 may be used as a source address for this host on this network; other addresses within 0.0.0.0/8 may be used to refer to specified hosts on this network [RFC1700, page 4].

A user on Reddit writes, this announcement will probably get “the same reaction when 1.1.1.1 and 1.0.0.1 became available, and AT&T blocked it ‘by accident’ or most equipment vendors or major ISP will use 0.0.0.0/8 as a loopback interface or test interface because they never thought it would be assigned to anyone.

Another user on Elegant treader writes, “I could actually see us successfully inventing, and implementing, a multiverse concept for ipv4 to make these 32 bit addresses last another 40 years, as opposed to throwing these non-upgradable, hardcoded v4 devices out”.

Another writes, if they would have “taken IPv4 and added more bits – we might all be using IPv6 now”. The user further mentions, “Instead they used the opportunity to cram every feature but the kitchen sink in there, so none of the hardware vendors were interested in implementing it and the backbones were slow to adopt it. So we got mass adoption of NAT instead of mass adoption of IPv6”.

A user explains, “A single /8 isn’t going to meaningfully impact the exhaustion issues IPv4 faces. I believe it was APNIC a couple of years ago who said they were already facing allocation requests equivalent to an /8 a month”.

It’s part of the reason hand-wringing over some of the “wasteful” /8s that were handed out to organizations in the early days is largely pointless. Even if you could get those orgs to consolidate and give back large useable ranges in those blocks, there’s simply not enough there to meaningfully change the long term mismatch between demand and supply”, the user further adds.

To know about these developments in detail, watch Dave Taht’s keynote video on YouTube:

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