Internet DRAFT - draft-schoen-intarea-unicast-lowest-address
draft-schoen-intarea-unicast-lowest-address
Internet Engineering Task Force S.D. Schoen
Internet-Draft J. Gilmore
Updates: 1122, 1812, 3021 (if approved) D. Täht
Intended status: Standards Track IPv4 Unicast Extensions Project
Expires: 1 July 2024 M. Karels
29 December 2023
Unicast Use of the Lowest Address in an IPv4 Subnet
draft-schoen-intarea-unicast-lowest-address-05
Abstract
With ever-increasing pressure to conserve IP address space on the
Internet, it makes sense to consider where relatively minor changes
can be made to fielded practice to improve numbering efficiency. One
such change, proposed by this document, is to increase the number of
unicast addresses in each existing subnet, by redefining the use of
the lowest-numbered (zeroth) host address in each IPv4 subnet as an
ordinary unicast host identifier, instead of as a duplicate segment-
directed broadcast address.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 1 July 2024.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Background and Current Standards . . . . . . . . . . . . . . 3
2.1. Assumptions About the Lowest Host Address by Remote
Systems . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Multicast Addresses; Point-to-Point Links . . . . . . . . 6
2.3. Current Limitations on Subnet-Directed Broadcast
Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6
2.4. Comparison to IPv6 Behavior . . . . . . . . . . . . . . . 6
3. Change to Interpretation of the Lowest Address . . . . . . . 7
3.1. Link-Layer Interaction . . . . . . . . . . . . . . . . . 7
3.2. Recommendations . . . . . . . . . . . . . . . . . . . . . 8
3.2.1. "Requirements for Internet Hosts -- Communication
Layers" [RFC1122] . . . . . . . . . . . . . . . . . . 8
3.2.2. "Requirements for IP Version 4 Routers" [RFC1812] . . 8
3.3. Example . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.4. Compatibility and Interoperability . . . . . . . . . . . 11
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Normative References . . . . . . . . . . . . . . . . . . 12
7.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Implementation Status . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
This document provides history and rationale to change the
interpretation of the lowest address in each IPv4 subnet from an
alternative broadcast address (or "network address") to an ordinary
assignable host address, and updates requirements for hosts and
routers accordingly. The decision taken in 1989 to reserve two forms
instead of one for local IPv4 segment broadcasts is no longer
necessary because of the obsolescence and disappearance of the
software that motivated it. Unreserving the lowest address provides
an optional extra IPv4 host address in every subnet, Internet-wide,
alleviating some of the pressure of IPv4 address exhaustion.
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1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Background and Current Standards
IPv4 has long supported several mechanisms for broadcasting
communications to every station on a network. One form of broadcast
in IPv4 is "segment-directed broadcast" in which a broadcast is
addressed to every station on a particular network (identified by its
network number). Current standards reserve a huge number of
addresses for this: not just one, but two, addresses per subnet,
Internet-wide.
The standard broadcast address on a subnet is the address whose "host
part" consists of all ones in binary. For example, in a 24-bit
subnet that starts at 192.168.3.0, the address 192.168.3.255 is the
standard broadcast address. [RFC1122][RFC0894]
In addition to the standard broadcast address, RFC 1122 (October
1989) acknowledged a then-current implementation behavior in "4.2BSD
Unix and its derivatives, but not 4.3BSD", whereby those operating
systems "use non-standard broadcast address forms, substituting 0 for
-1". (Note that there was no standard for IP broadcast when 4.2BSD
was released in August 1983, more than a year prior to [RFC0919].
The -1 form was first proposed in [IEN212] in 1982 but was not yet a
standard.) According to RFC 1122 and its successors, Internet hosts
are expected to "recognize and accept [...] these non-standard
broadcast addresses as the destination address of an incoming
datagram", and not otherwise use them to identify Internet hosts.
RFCs 1812 [RFC1812] (sections 4.2.3.1 and 5.3.5), and 3021 [RFC3021]
(sections 2.2, 3.1, and 3.3) reiterate and further expand on this
requirement.
This non-standard broadcast address is the address whose "host part"
consists of all zeroes. (The quantity of zeroes depends, in present-
day terminology, on the applicable subnet mask.) This address is the
lowest expressible address within any particular numbered network.
Following computer science tradition, it may also be referred to as
the "zeroth address" of its respective subnet. Many sources also
refer to it as the "network address" of the subnet, because the
subnet as a whole (pre-CIDR) was commonly referred to by its lowest
address, while under CIDR the subnet as a whole is commonly referred
to by its lowest address plus a prefix length.
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The address triple syntax used in RFC 1122 looks unusual to modern
eyes. These triples included the "{ network number, subnet number,
host number }". The notation also used two's complement binary
notation in referring to a host number "-1" as containing all binary
1-bits. After the widespread adoption of CIDR [RFC4632], network
numbers no longer have an "address class" definition based on their
high-order bits, and there is no distinction between a network number
and a subnet number (except locally at the router where individual
subnets are being routed). Instead, following RFC 4632, IPv4 network
addresses are denoted with a dotted-decimal format containing one to
four positive 8-bit integers, a slash, and a whole count of the bits
in the network number portion, the so-called CIDR notation,
reportedly devised by Phil Karn. So for example 192.1.2.0/28 has a
28-bit network number and a 4-bit host number (32 address bits total,
minus those 28 bits). All of the bits of that particular host number
are zero (because the whole fourth dotted number is 0), and thus the
interpretation of this address would be affected by this document.
We will use both notations as convenient. Where RFC 1122 and its
successors use the terms "0 form" and "-1 form", we may refer
respectively to "all-zeros" and "all-ones" host numbers, since every
bit in the binary representation of these two host numbers has the
value 0 or 1, respectively.
4.2BSD, the operating system to whose behavior RFC 1122 required
deference, was the first BSD operating system full release to include
TCP/IP support; it was released in 1983. Its successor, 4.3BSD, was
released in 1986, which is why RFC 1122 could already confirm that
the broadcast behavior had been changed. See [BSDHIST]. RFC 1812
calls the old behavior "obsolete" in 1995, and RFC 3021 reiterates
that it is "obsolete" in 2000, although both express the idea that
the lowest address must continue to be reserved for historical
reasons.
All subsequent operating systems used on the Internet implement the
standard all-ones form of the broadcast address and use it by
default. Continuing to reserve the non-standard all-zeroes form
wastes one IPv4 address per subnet. No known operating system
generates IP broadcasts in this format by default today, and
documentation consistently encourages network administrators and
software developers to use the standard form. The IPv4 protocol does
not benefit from having two different broadcast addresses with the
same functionality in every subnet, and the non-standard form has
always been reserved primarily for backwards compatibility with
systems that have not existed for decades on the Internet.
As IPv4 addresses were not perceived as particularly scarce through
the 1980s, the prospect of wasting tens of millions of otherwise
assignable addresses in order to achieve backwards compatibility with
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a particular operating system appeared reasonable. Today, those
addresses are clearly valuable and could be put to good use as
identifiers of Internet hosts in a time of IPv4 numbering resource
exhaustion.
2.1. Assumptions About the Lowest Host Address by Remote Systems
In general, under CIDR [RFC4632], only hosts and routers on a network
segment know that segment's netmask with certainty. Remote parts of
the Internet are already expected not to make assumptions about
whether or not a particular address is a broadcast address, since
that determination is already only meaningful for devices connected
to the segment containing that address. This document does not
change any of these things. Thus, if the behavior of devices on a
particular network segment has been updated in accordance with this
memo, the lowest address on that segment can already be addressed by
hosts elsewhere on the Internet without any changes to their own
software.
[RFC1812] noted in section 4.2.3.1 that whether a reserved address is
treated specially at all depends on one's vantage point on the
network:
| [a] router obviously cannot recognize addresses of the form {
| <Network-prefix>, 0 } if the router has no interface to that
| network prefix. In that case, the rules of the second bullet
| [requiring a packet to be discarded] do not apply because, from
| the point of view of the router, the packet is not an IP broadcast
| packet.
It also noted in section 5.3.5.2 that
| in view of CIDR, such [packets addressed to broadcast addresses of
| distant networks] appear to be host addresses within the network
| prefix; we preclude inspection of the host part of such network
| prefixes.
To the extent that software continues to make assumptions about IP
network classes today, it is out of compliance with RFC 4632. Ever
since the adoption of CIDR in RFC 1519, it has been unknowable
whether or to what extent the remote network would internally
aggregate or deaggregate routes that were visible elsewhere on the
Internet. Therefore, Internet hosts and routers MUST NOT assume that
an IPv4 address on a remote network, other than 0.0.0.0, is invalid,
unroutable, or inaccessible merely because it ends with a particular
number of zeroes. In keeping with the Internet's end-to-end
principle, decisions about possible invalidity of otherwise routable
addresses belong as close to the endpoints as possible.
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2.2. Multicast Addresses; Point-to-Point Links
Multicast addresses, as defined by RFC 1112 [RFC1112], do not have a
network part and host part, nor do they have a netmask or CIDR prefix
length. IPv4 multicast addresses, except 224.0.0.0 (which is
"guaranteed not to be assigned to any group" by RFC 1112), could
always end with any number of zeroes, and have never had any form of
directed broadcast address.
[RFC3021], section 2.1, standardized that, in a point-to-point link
using a 31-bit netmask, the all-zero and all-one forms of the host-
part of the address MUST both be treated as unicast ("host")
addresses.
The present document does not change the interpretation of multicast
addresses or 31-bit subnet addresses in any way.
2.3. Current Limitations on Subnet-Directed Broadcast Addresses
Sending packets to a subnet-directed broadcast address is still
generally useful in today's Internet, but only for nodes attached
directly to that subnet. [RFC2644] discouraged routers from
forwarding such packets, to reduce their use in amplifying denial-of-
service attacks, so they often cannot be received when sent from
distant hosts. Many hosts today ignore ICMP packets sent as
broadcasts, so a directed broadcast ping is no longer a reliable
means of enumerating all hosts attached to a network. As
Informational [RFC6250] notes, "broadcast can only be relied on
within a link".
2.4. Comparison to IPv6 Behavior
In IPv6 there are no reserved per-segment broadcast addresses (or,
indeed, any broadcast addresses whatsoever). Instead, IPv6 hosts can
address all hosts on a network segment by using the link-local
multicast group address ff02::1 [RFC4291], which, for example,
produces a multicast Ethernet frame when transmitted over an
Ethernet-like link [RFC2464]. The lowest address on a subnet is,
however, reserved in IPv6 by Section 2.6.1 of RFC 4291 [RFC4291] for
the Subnet-Router address (a means of addressing "any router" on an
indicated subnet).
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3. Change to Interpretation of the Lowest Address
The purpose of this document is to designate the all-zeros address in
each subnet as a unicast address. All such addresses are now
available for general non-broadcast use, treated identically to all
host addresses in the subnet besides the "all-ones" broadcast
address. This document therefore eliminates an element of the IPv4
protocol's historical adaptation to 4.2BSD's behavior. All hosts
SHOULD continue to recognize and accept only the all-ones form of the
IPv4 subnet broadcast address.
Host software that intends to transmit a segment-directed broadcast
packet in an IPv4 network MUST use only the all-ones form as the
destination address of the packet.
An IPv4 datagram containing a source or destination that is equal to
the all-zeroes form of the local broadcast address SHOULD be treated,
by both hosts and routers, as a normal unicast datagram; it SHOULD
NOT be treated as a local broadcast datagram.
Host software SHOULD allow a network interface to be configured with
the lowest address on a subnet. A host with such an address
configured MUST use this assigned address as a source address for
datagrams just as it would with any other assigned interface address,
and MUST recognize a datagram sent to that address as addressed to
itself. Host software SHOULD be capable of generating unicast
packets to the lowest address on a subnet when so requested by an
application, and MUST encapsulate such packets into link-layer
unicast frames when transmitted on a link layer that distinguishes
unicast and broadcast.
Clients for autoconfiguration mechanisms such as DHCP [RFC2131]
SHOULD accept a lease or assignment of the lowest address whenever
the underlying operating system is capable of accepting it. Servers
for these mechanisms SHOULD assign this address when so configured.
The network operator of each subnet retains the discretion to number
hosts on that subnet with, or without, the use of the lowest address,
based on local conditions.
3.1. Link-Layer Interaction
The link layer always indicates to the IP layer whether or not a
datagram was transmitted as a broadcast at the link layer. Hosts
MUST continue to follow the RFC 1122 rule about link-layer broadcast
indications:
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| A host SHOULD silently discard a datagram that is received via a
| link-layer broadcast [...] but does not specify an IP multicast or
| broadcast destination address.
This rule is, among other things, intended to avoid broadcast storms.
This document now defines the lowest address as a non-broadcast
address. Therefore, a host SHOULD silently discard a datagram
received via a link-layer broadcast whose destination address is the
lowest IPv4 address in a subnet. This is true even if the interface
on which the host received that datagram uses the lowest address as a
unicast IPv4 address.
3.2. Recommendations
The considerations presented in this document affect other published
work. This section details the updates made to other documents.
3.2.1. "Requirements for Internet Hosts -- Communication Layers"
[RFC1122]
The new section numbered 3.2.1.3 (h) which was added by RFC 3021 is
replaced with:
| (h) { <Network-number>, <Subnet-number>, 0 }
|
| An ordinary unicast ("host") address in the subnet. May be used
| as either a source or destination address. If a link-level
| broadcast packet is received with this address (or any other
| unicast address) as its destination, it MUST be silently
| discarded. Such a packet may be sent by long-obsolete hosts on
| the local network.
|
| In applications using CIDR notation [RFC4632], this address, or
| any other address in the subnet, may also be used together with a
| prefix length to refer to the entire subnet.
3.2.2. "Requirements for IP Version 4 Routers" [RFC1812]
The new section (numbered 4.2.2.11 (f)) added by RFC 3021 is replaced
by:
| (f) { <Network-prefix>, 0 }
|
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| An ordinary unicast ("host") address in the subnet. May be used
| as either a source or destination address. If a link-layer
| broadcast packet is received with this address (or any other
| unicast address) as its destination, it MUST be silently
| discarded. Such a packet may be sent by long-obsolete hosts on
| the local network.
|
| In applications using CIDR notation [RFC4632], this address, or
| any other address in the subnet, may also be used together with a
| prefix length to refer to the entire subnet.
The first paragraph on page 49 (which appears after section 4.2.2.11
(e) in the original RFC 1812, or after section 4.2.2.11 (f) in RFC
1812 as modified by RFC 3021) is changed from this original text
| IP addresses are not permitted to have the value 0 or -1 for the
| <Host-number> or <Network-prefix> fields except in the special
| cases listed above. This implies that each of these fields will
| be at least two bits long.
|
| DISCUSSION
|
| Previous versions of this document also noted that subnet numbers
| must be neither 0 nor -1, and must be at least two bits in length.
| In a CIDR world, the subnet number is clearly an extension of the
| network prefix and cannot be interpreted without the remainder of
| the prefix. This restriction of subnet numbers is therefore
| meaningless in view of CIDR and may be safely ignored.
to this new text
| Unicast IP addresses are permitted to have the value 0 for the
| <Host-number> field, and may have the value -1 in the special
| cases listed above. There is no requirement that the <Host-
| number> field be any particular length. In some cases using CIDR
| notation, a host may be designated with a /32 suffix (e.g.
| 192.0.2.34/32), indicating that the specific host rather than its
| subnet is being described.
|
| DISCUSSION
|
| Previous versions of this document also noted that subnet numbers
| must be neither 0 nor -1, and must be at least two bits in length.
| Other versions required that <Network-prefix> fields must be
| neither 0 nor -1, and must be at least two bits long.
|
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| Now that the Internet has fully transitioned to CIDR routing,
| there are no original classful <Network-number>s to be
| distinguished from <Subnet-numbers>. Each address only has a
| <Network-prefix> based on its network mask (or equivalently, the
| CIDR suffix specifying how many bits are in the <Network-prefix>).
| The former restrictions on subnet numbers and their sizes are
| meaningless in view of CIDR and are hereby repealed. For example,
| a route to 0.0.0.0/6 or even 0.0.0.0/1 is a viable CIDR route (for
| the aggregation of the blocks 0/8, 1/8, 2/8, and 3/8; or for the
| entire lower half of the IPv4 address space) and should not be
| considered invalid. 0.0.0.0/0 is standardized to mean "all unicast
| IPv4 addresses", e.g. in a default route, by section 5.1 of
| [RFC4632], which MUST also continue to work.
Sections 4.2.3.1 (2) and (4) are replaced with:
| (2) SHOULD silently discard on receipt (i.e., do not even deliver
| to applications in the router) any packet addressed to 0.0.0.0.
| If these packets are not silently discarded, they MUST be treated
| as IP broadcasts (see Section [5.3.5]). There MAY be a
| configuration option to allow receipt of these packets. This
| option SHOULD default to discarding them.
|
| A packet addressed to { <Network-prefix>, 0 } is an ordinary
| unicast packet, and MUST be treated as such.
|
| (4) SHOULD NOT originate datagrams addressed to 0.0.0.0. SHOULD
| allow for the generation of datagrams addressed to {<Network-
| prefix>, 0 } since that is now defined as an ordinary unicast
| adddress.
3.3. Example
The only IPv4 broadcast address for 192.168.42.0/24 is 192.168.42.255
(the all-ones or "-1" host number). 192.168.42.0 (the all-zeroes or
"0" host number) was formerly a second broadcast address on that
subnet, but is now a unicast address.
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The fact that the address identifier 192.168.42.0 can refer both to a
network (as when it is regarded as a "network address" with or
without an explicit prefix length) and to a specific host
192.168.42.0 is not unusual. Similarly, referring to a subnet as
192.168.42.0/24 and configuring a particular interface on that subnet
as 192.168.42.0/24 is also not unusual. Computer scientists normally
count all sorts of things starting at the zeroth (lowest) element in
a sequence.[EWD831] For example, the initial element in an array is
likely to be stored at a memory address equal to the memory address
of the array itself.[ARRAY] Similarly, IPv4 hosts in a subnet MAY be
enumerated starting with an address that matches the address of the
subnet itself.
Similarly, the only IPv4 broadcast address for the subnet
192.168.42.96/28 is 192.168.42.111. The address 192.168.42.96 MAY be
assigned to an individual host on this network.
3.4. Compatibility and Interoperability
Many deployed systems follow older Internet standards in not allowing
the lowest address in a network to be assigned or used as a source or
destination address. Assigning this address to a host may thus make
it inaccessible by some devices on its local network segment.
Network operators considering assigning this address to a host should
investigate their own network environments to determine whether their
interoperability requirements will be met. Interoperability with
these addresses is likely to improve over time, following the
publication of this document.
Prior standards required hosts and routers to ignore, and to refrain
from generating, non-broadcast datagrams from or to the lowest
address. So when a single network contains a device that has been
assigned the lowest address as specified by this document, along with
one or more devices that follow the traditional behavior, the
traditional devices will not be able to communicate with the lowest-
address device at all. Other sorts of malfunctions are unlikely,
because the former standards (RFC 1122) required traditional hosts to
drop any unicast packet addressed to the secondary broadcast address
that they implemented at the lowest address.
4. IANA Considerations
This memo includes no request to IANA.
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5. Security Considerations
The behavior change specified by this document could produce security
concerns where two devices, or two different pieces of software on a
single host, or a software application and a human user, follow
divergent interpretations of the lowest address on a network. For
example, this could lead to errors in the specification or
enforcement of rules about Internet hosts' connectivity to one
another, or their right to access resources.
Firewall rules that assume that the lowest address on a subnet cannot
be addressed SHOULD be updated to take into account that it can be
addressed, so as to avoid either unintentionally allowing or
unintentionally forbidding connections involving it. Other security,
monitoring, or logging systems that treat the lowest address as an
inaccessible bogon address SHOULD likewise be updated.
Host software SHOULD make the distinction between lowest-address
(considered individually) and subnet (considered as a group) clear to
users, where this distinction is relevant and could be a subject of
confusion.
6. Acknowledgements
This document directly builds on prior work by Dave Täht and John
Gilmore as part of the IPv4 Unicast Extensions Project.
7. References
7.1. Normative References
[RFC0919] Mogul, J., "Broadcasting Internet Datagrams", STD 5,
RFC 919, DOI 10.17487/RFC0919, October 1984,
<https://www.rfc-editor.org/info/rfc919>.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.
[RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers",
RFC 1812, DOI 10.17487/RFC1812, June 1995,
<https://www.rfc-editor.org/info/rfc1812>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC4632] Fuller, V. and T. Li, "Classless Inter-domain Routing
(CIDR): The Internet Address Assignment and Aggregation
Plan", BCP 122, RFC 4632, DOI 10.17487/RFC4632, August
2006, <https://www.rfc-editor.org/info/rfc4632>.
7.2. Informative References
[ARRAY] Wikipedia, "C Programming Language: Array-pointer
interchangeability", <https://en.wikipedia.org/wiki/C_(pro
gramming_language)#Array%E2%80%93pointer_interchangeabilit
y>.
[BSDHIST] Wikipedia, "History of the Berkeley Software
Distribution", <https://en.wikipedia.org/wiki/
History_of_the_Berkeley_Software_Distribution>.
[EWD831] Dijkstra, E.W., "Why Numbering Should Start at Zero",
August 1982,
<https://www.cs.utexas.edu/users/EWD/transcriptions/
EWD08xx/EWD831.html>.
[IEN212] Gurwitz, R. and R. Hinden, "IP - Local Area Network
Addressing Issues", IEN 212, September 1982,
<https://www.postel.org/ien/pdf/ien212.pdf>.
[RFC0894] Hornig, C., "A Standard for the Transmission of IP
Datagrams over Ethernet Networks", STD 41, RFC 894,
DOI 10.17487/RFC0894, April 1984,
<https://www.rfc-editor.org/info/rfc894>.
[RFC1112] Deering, S., "Host extensions for IP multicasting", STD 5,
RFC 1112, DOI 10.17487/RFC1112, August 1989,
<https://www.rfc-editor.org/info/rfc1112>.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<https://www.rfc-editor.org/info/rfc2131>.
[RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet
Networks", RFC 2464, DOI 10.17487/RFC2464, December 1998,
<https://www.rfc-editor.org/info/rfc2464>.
[RFC2644] Senie, D., "Changing the Default for Directed Broadcasts
in Routers", BCP 34, RFC 2644, DOI 10.17487/RFC2644,
August 1999, <https://www.rfc-editor.org/info/rfc2644>.
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[RFC3021] Retana, A., White, R., Fuller, V., and D. McPherson,
"Using 31-Bit Prefixes on IPv4 Point-to-Point Links",
RFC 3021, DOI 10.17487/RFC3021, December 2000,
<https://www.rfc-editor.org/info/rfc3021>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.
[RFC6250] Thaler, D., "Evolution of the IP Model", RFC 6250,
DOI 10.17487/RFC6250, May 2011,
<https://www.rfc-editor.org/info/rfc6250>.
Appendix A. Implementation Status
The behavior specified in this document has been implemented by
OpenBSD since version 4.7, released in May 2010.
The behavior specified in this document has been implemented by the
Linux kernel since version 5.14, released in August 2021.
This behavior has also been implemented in the OS releases Fedora 35
(released in November 2021) and Ubuntu 22.04 (released in April
2022). In addition, nodes that run Fedora 34 and install the
standard online updates also implement this feature. This behavior
has also been included in OpenWrt as of OpenWrt 23.05 (released in
October 2023).
This behavior was an option in NetBSD since 1999 (via the
net.inet.ip.hostzerobroadcast sysctl), and became the default
behavior in 2016.
This behavior has also been implemented in FreeBSD since release 13.1
of May 2022. The FreeBSD, OpenBSD, NetBSD, and Linux implementations
interoperate successfully.
Popular embedded Internet-of-Things environments such as RIOT and
FreeRTOS have implemented this behavior for many years.
This behavior is optionally available in Android 13 (released in
August 2022) when using the android13-5.15 kernel version, and is
implemented by default in Android 14 beta.
Compatiblity with lowest-address assignment may be typical of many
DHCP implementations (because the lowest address special case has
often been handled at the kernel level). If the underlying operating
system supports lowest-address assignment, the final official ISC
DHCP release (4.4.3) supports lowest-address allocation as both
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client and server, as do Busybox DHCP udhcpc/udhcpd (release 1.1.15),
and ISC Kea (which currently includes only a DHCP server
implementation).
Authors' Addresses
Seth David Schoen
IPv4 Unicast Extensions Project
San Francisco, CA
United States of America
Email: schoen@loyalty.org
John Gilmore
IPv4 Unicast Extensions Project
PO Box 170640-rfc
San Francisco, CA 94117-0640
United States of America
Email: gnu@rfc.toad.com
David M. Täht
IPv4 Unicast Extensions Project
Half Moon Bay, CA
United States of America
Email: dave@taht.net
Michael J. Karels
Eden Prairie, MN
United States of America
Email: rfc@karels.net
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