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Unicast Use of the Formerly Reserved 0/8

Document Type Active Internet-Draft (individual)
Authors Seth David Schoen , John IETF Gilmore , David M. Täht
Last updated 2023-12-29
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Internet Engineering Task Force                              S.D. Schoen
Internet-Draft                                                J. Gilmore
Updates: 1122, 1812, 2827, 3704 (if approved)                    D. Täht
Intended status: Standards Track         IPv4 Unicast Extensions Project
Expires: 1 July 2024                                    29 December 2023

                Unicast Use of the Formerly Reserved 0/8


   This document redesignates 0/8, the lowest block in the IPv4 address
   space, so that this space is no longer reserved.  It asks
   implementers to make addresses in this range fully usable for unicast
   use on the Internet.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   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 (
   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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   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
   2.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Present situation . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Change in Status of 0/8 . . . . . . . . . . . . . . . . . . .   4
     4.1.  No Change to Interpretations of . . . . . . . . .   4
   5.  Other Existing Uses of 0/8  . . . . . . . . . . . . . . . . .   5
   6.  Compatibility and Interoperability  . . . . . . . . . . . . .   6
   7.  Unofficial uses of 0/8  . . . . . . . . . . . . . . . . . . .   8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   11. Normative References  . . . . . . . . . . . . . . . . . . . .   9
   12. Informative References  . . . . . . . . . . . . . . . . . . .  11
   Appendix A.  Implementation Status  . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   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 allow the use of more
   than 16 million historically reserved addresses at the bottom of the
   IPv4 address space.

   This document provides history and rationale to change the status of
   the "0/8" or "zeroth" region of the IPv4 address space (historically
   known as "unspecified network" or "this network") from reserved to
   unreserved.  These addresses are already usable for unicast traffic
   in some popular TCP/IP implementations today.  Standardization as
   unicast addresses will eventually allow them to be later deployed by
   Internet stewardship organizations to relieve address space scarcity.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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2.  Background

   The early Internet reserved many kinds of IPv4 addresses for special
   purposes.  One important such designation involves every IPv4 address
   beginning with the octet 0 (now "0/8"); all these addresses were
   designated for use in potential device autoconfiguration features
   that were to use ICMP messages [RFC0792].  This function was
   eventually entirely supplanted by other protocols, which, in IPv4,
   now use only the single address

   Autodiscovery of a network-provided configuration came to be handled
   for IPv4 by DHCP [RFC2131], and formerly by its predecessors BOOTP
   [RFC0951] and RARP [RFC0903].  In modern practice, the source address
   of a device seeking an IPv4 configuration from the local network is
   indicated with a link-layer broadcast in which the network-layer
   source address and the network-layer destination address is

   The reservation of 0/8, despite its obsolescence, has been reiterated
   in all subsequent IPv4 address allocation RFCs and IANA documents.
   By 1989, [RFC1122], section, for example, noted that this
   mechanism was already obsolete, even as section continued to
   expressly prohibit the use of network number 0 for other purposes.

   The single special address acquired a variety of related
   meanings including "unspecified address", "unknown address", "address
   not set", "address not applicable", etc., while means "the
   default route", which contains every IPv4 host.  This single address
   has remained sufficient for these purposes.

3.  Present situation

   Today, 0/8 addresses (except for the special address are no
   longer used in any autoconfiguration protocols.  All of this
   functionality is handled using other distinctly-specified mechanisms
   and address space, both in IPv4 and IPv6.

   The designation of 0/8 as reserved address space is tracked by IANA
   in the IPv4 Special-Purpose Address Registry [IANA4], as provided for
   by RFC 6890 [RFC6890].  No known software makes use of this address
   space in the headers of IPv4 packets transmitted over the wire.
   While some software already treats it as a potentially valid address,
   the most common behavior by host and router software when
   encountering any address within 0/8 is to reject it as a Martian
   address.  These and all other known uses are discussed in the
   sections "Other Existing Uses of 0/8" and "Compatibility and
   Interoperability", below.

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   Since this address space has no existing widespread practical use or
   interpretation, it can be used for other purposes and help alleviate
   the shortage of IPv4 addresses.  This memo therefore unreserves it,
   redesignating it as unassigned unicast addresses, available for
   potential global unicast or other allocation.

4.  Change in Status of 0/8

   The purpose of this document is to make addresses in the range 0/8
   available for active unicast use on the public Internet.  This
   includes supporting them for numbering and addressing networks and
   hosts, like any other unicast address.

   As an exception, the address retains its existing special
   meanings, as described in the subsection "No Change to
   Interpretations of".

   Host and router software SHOULD treat addresses in the 0/8 range,
   except the host address, in the same way that they would
   treat other unicast IPv4 addresses.  Software SHOULD be capable of
   accepting datagrams from, and generating datagrams to, addresses
   within this range.

   Clients for autoconfiguration mechanisms such as DHCP [RFC2131]
   SHOULD accept a lease or assignment of an address within 0/8, except
   the host address, whenever the underlying operating system is
   capable of accepting it.

4.1.  No Change to Interpretations of

   The unqualified address or the individual host address has many special meanings which are described in a section
   "Other Existing Uses of 0/8", below.  This document does not make
   this all-zero address an individually valid unicast address, and it
   should still not be taken to identify an individual device.  As
   described in prior Internet standards, the address MUST NOT
   be assigned to an individual interface.  This address is valid to
   appear in both source and destination addresses, with special
   meanings, in protocols already defined or to be defined in the

   The network identifier also continues to be used to refer
   to an IPv4 default route (a route which matches any Internet host).
   This is not inconsistent with the use of explicit routes to
   individual networks within  Existing CIDR-based routing
   logic is readily capable of distinguishing an object like
   (a route referring to a specific /8 whose leftmost octet is always 0)
   from one like (a route including to any IPv4 host); in

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   current routing practice, the default route already always
   overlaps every more-specific route, regardless of how many zero bits
   appear at the beginning of a more-specific route's destination.

   For avoidance of doubt, we note that all routing implementations MUST
   permit routes to overlap, and MUST distinguish the default route from a more-specific CIDR route such as or, as well as from a leading-zero-octet route such as  These distinctions are already implied by [RFC4632],
   section 3.1 (since neither "n" nor "x" is ever stated to be nonzero),
   and sections 5.1 and 5.2 (describing and requiring generality in the
   treatment of arbitrary routes, including the default route).

5.  Other Existing Uses of 0/8

   There are a handful of other uses of 0/8 with special meanings in
   existing Internet protocols and standards.

   A large number of protocols and environments use the special address to mean "unspecified", "unknown", "unset", "not applicable",
   "any address", "no address", or, as, the default route
   containing every IPv4 network.  (Two examples, among dozens, are
   [RFC2131]'s use of in DHCP packets to mean "my IP source
   address is unknown" and [RFC4541]'s use of to mean "proxied
   IGMP membership report from a non-Querier".)

   All these uses of the address are unchanged by this memo.
   Due to its variety of special meanings, the address MUST NOT
   be allocated exclusively to a specific organization or network.
   Existing standards significantly constrain, but do not preclude,
   circumstances in which it may appear on the wire.

   There are three known non-unicast uses of the 0/8 block as a whole in
   the RFC series.

   *  RFC 3338 [RFC3338] (an IPv6 transition mechanism) used 0/8
      addresses as synthetic addresses representing surrogate IPv6
      addresses, but this practice has already been deprecated by
      [RFC6535], which indicated that this transition mechanism should
      switch to RFC 1918 private addresses.

   *  RFC 7453 [RFC7453] (an MPLS-related SNMP MIB definition) overloads
      the meaning of addresses in 0/8 by designating them as local
      identifiers, contrasting with IPv4 addresses.  Before production
      use of 0/8 on the global Internet occurs, this MIB should be
      updated to provide a separate field for local identifiers and to
      deprecate the old semantics.

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   *  RFC 6235 [RFC6235] and RFC 8932 [RFC8932] both provide mechanisms
      for anonymizing network flow datasets that can map addresses into
      0/8 in order to obscure them.  Implementers SHOULD take into
      account that source addresses in the future may already lie in
      this range and will still require anonymization; an IPv4 address
      SHOULD NOT be assumed to have been anonymized already merely
      because it is within 0/8.

6.  Compatibility and Interoperability

   Older Internet standards counseled implementations in varying ways to
   reject packets from, and not to generate packets to, addresses within

   Among several standards calling for this behavior, RFC 1122, section, and RFC 1812, section, say that hosts and routers,
   respectively, MUST NOT send packets using these addresses, outside of
   configuration-discovery processes.  RFC 1122 implies hosts MUST
   discard, and RFC 1812 implies routers SHOULD NOT forward, packets
   whose source address is within 0/8.

   RFC 3704 [RFC3704] (BCP 84) cites RFC 2827 [RFC2827] (BCP 38) in
   asking providers to filter based on source address:

   |  RFC 2827 recommends that ISPs police their customers' traffic by
   |  dropping traffic entering their networks that is coming from a
   |  source address not legitimately in use by the customer network.
   |  The filtering includes but is in no way limited to the traffic
   |  whose source address is a so-called "Martian Address" - an address
   |  that is reserved, including any address within,
   |, or

   Other RFCs such as 3964, 4380, and 6491 have reiterated specific
   lists of Martian ranges for other purposes, rather than referring to
   the subsequently-created IANA Special-Purpose Address Registry.  We
   encourage future RFC authors and implementers to refer to the
   Special-Purpose Address Registry rather than explicitly providing or
   using a list of reserved addresses within their documentation.

   In this context, RFC 3704 specifies filtering of these addresses as
   source (not destination) addresses at a network ingress point as a
   countermeasure against forged source addresses, limiting forwarded
   packets' source addresses to only the set which have been actually
   assigned to the customer's network.  The RFC's mention of these
   "Martian Addresses" is based on the assumption that they could never
   be legitimately in use by the customer network.

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   Because the 0/8 address space is no longer reserved as a whole, an
   address within this space is no longer inherently a "Martian"
   address.  Both hosts and routers MUST NOT hard-code a policy of
   always rejecting such addresses.  Hosts and routers SHOULD NOT be
   configured to apply Martian address filtering to any packet solely on
   the basis of its reference to a source (or destination) address in
   0/8.  Maintainers of lists of "Martian addresses" MUST NOT designate
   addresses from this range as "Martian".  As noted elsewhere, the
   address retains its special meaning, but is also not a
   "Martian" address.

   The filtering recommended by RFC 3704 is designed for border routers,
   not for hosts.  To the extent that an ISP had allocated an address
   range from within 0/8 to its customer, RFC 3704 would already not
   require packets with those source addresses to be filtered out by the
   ISP's border router.

   Since deployed implementations' willingness to accept 0/8 addresses
   as valid unicast addresses varies, a host to which an address from
   this range has been assigned may also have a varying ability to
   communicate with other hosts.

   Such a host might be inaccessible by some devices either on its local
   network segment or elsewhere on the Internet, due to a combination of
   host software limitations or reachability limitations in the network.
   IPv4 unicast interoperability with 0/8 can be expected to improve
   over time following the publication of this document.  Before or
   after allocations are eventually made within this range,
   "debogonization" efforts for allocated ranges can improve
   reachability to the whole address block.  Similar efforts have
   already been done by Cloudflare on [Cloudflare], and by RIPE
   Labs on 1/8 [RIPElabs18], 2a10::/12 [RIPElabs2a1012], and 128.0/16
   [RIPElabs128016].  The Internet community can use network probing
   with any of several measurement-oriented platforms to investigate how
   usable these addresses are at any particular point in time, as well
   as to localize medium-to-large-scale routing problems.  (Examples are
   described in [Huston], [NLNOGRing], and [Atlas].)  Any network
   operator to whom such addresses are made available by a future
   allocation will have to examine the situation in detail to determine
   how well its interoperability requirements will be met.

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7.  Unofficial uses of 0/8

   Some organizations may be using portions of 0/8 internally as RFC
   1918-type private-use address space, for example for internal
   communications within datacenters.  We currently have no publicly-
   documented examples of this practice.  However, future allocations of
   0/8 could result in use of this space on the public Internet in ways
   that overlap these unofficial private-use addresses, creating
   ambiguity about whether a particular host intended to use such an
   address to refer to a private or public network (since the address
   would then have two distinct interpretations with different
   addressing scopes).  Among other unintended outcomes, hosts or
   firewalls that have extended greater trust to other hosts based on
   their use of a certain unofficial network number (that was considered
   to imply presence on a LAN or within an organization) may eventually
   receive legitimate traffic from an external network to which this
   address space has been allocated.

   Operators of networks that are making unofficial uses of portions of
   0/8 may wish to plan to discontinue these uses and renumber their
   internal networks, or to request that IANA formally designate certain
   ranges as additional Private-Use areas.

8.  IANA Considerations

   This memo unreserves the address block 0/8.  It therefore requests
   IANA to update the IANA IPv4 Special-Purpose Address Registry [IANA4]
   by removing the entry for 0/8, whose existing authority is RFC 791
   [RFC0791], Section 3.2.  Additionally, it requests IANA to update the
   IANA IPv4 Address Space Registry by changing the entry for 000/8 from
   "IANA - Local Identification, 1981-09, RESERVED" to "Unallocated,
   Former IANA - Local Identification, [Date of this RFC], UNALLOCATED".
   Finally, IANA is requested to prepare for this address space to be
   addressed in the reverse DNS space in

   This memo does not effect a registration, transfer, allocation, or
   authorization for use of these addresses by any specific entity.
   This memo's scope is to require IPv4 software implementations to
   support the ordinary unicast use of addresses in the newly
   unallocated range through  During a
   significant transition period, it would only be prudent for the
   global Internet to use those addresses for experimental purposes such
   as de-bogonization testing.  After that transition period, a
   responsible entity such as IETF or IANA could later consider whether,
   how and when to allocate those addresses to entities or to other
   protocol functions.

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9.  Security Considerations

   The change specified by this document could create a period of
   ambiguity about historical and future interpretations of the meaning
   of host and network addresses in 0/8.  Some networks and hosts
   currently discard all IPv4 packets bearing these addresses, pursuant
   to statements in prior standards that packets containing these
   addresses have no agreed-upon meaning and ought not to be sent over
   the wire.

   Disparate filtering processes and rules at present, and in response
   to the adoption of this document, could make it easier for rogue
   network operators to hijack or spoof portions of this address space
   in order to send malicious traffic.

   Live traffic, accepted and processed by other devices, may
   legitimately originate from 0/8 addresses in the future.  Network
   operators, firewalls, and intrusion-detection systems may need to
   take account of this change in various regards, including so as to
   avoid permitting either more or less traffic from such addresses than
   they expected.

   Automated systems generating reports, and human beings reading those
   reports, SHOULD NOT assume that the use of a 0/8 source address
   indicates spoofing, an attack, or a new incompatible packet format.
   At the same time, they SHOULD NOT assume that the use of 0/8 is
   impossible or will be precluded by other systems' behavior.

   Since the Linux kernel has already defaulted to the specified
   behavior for two years (see "Implementation Status"), it is already
   possible for deployed systems to disagree about whether packets
   containing 0/8 may validly appear on the wire.  This document offers
   an opportunity to move to a new consensus in which implementations
   widely agree that these packets are potentially valid, while giving
   implementers considerable advance notice ahead of any future
   deployment of these addresses on the public Internet.

10.  Acknowledgements

   This document directly builds on prior work by Dave Täht and John
   Gilmore as part of the IPv4 Unicast Extensions Project.
   Acknowledgements of contributions to their drafts still need to be
   transposed here.

11.  Normative References

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   [IANA4]    Internet Assigned Numbers Authority, "IANA IPv4 Special-
              Purpose Address Registry",

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,

   [RFC0903]  Finlayson, R., Mann, T., Mogul, J., and M. Theimer, "A
              Reverse Address Resolution Protocol", STD 38, RFC 903,
              DOI 10.17487/RFC0903, June 1984,

   [RFC0951]  Croft, W. and J. Gilmore, "Bootstrap Protocol", RFC 951,
              DOI 10.17487/RFC0951, September 1985,

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
              May 2000, <>.

   [RFC3338]  Lee, S., Shin, M-K., Kim, Y-J., Nordmark, E., and A.
              Durand, "Dual Stack Hosts Using "Bump-in-the-API" (BIA)",
              RFC 3338, DOI 10.17487/RFC3338, October 2002,

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   [RFC3704]  Baker, F. and P. Savola, "Ingress Filtering for Multihomed
              Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March
              2004, <>.

   [RFC4541]  Christensen, M., Kimball, K., and F. Solensky,
              "Considerations for Internet Group Management Protocol
              (IGMP) and Multicast Listener Discovery (MLD) Snooping
              Switches", RFC 4541, DOI 10.17487/RFC4541, May 2006,

   [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, <>.

   [RFC6235]  Boschi, E. and B. Trammell, "IP Flow Anonymization
              Support", RFC 6235, DOI 10.17487/RFC6235, May 2011,

   [RFC6535]  Huang, B., Deng, H., and T. Savolainen, "Dual-Stack Hosts
              Using "Bump-in-the-Host" (BIH)", RFC 6535,
              DOI 10.17487/RFC6535, February 2012,

   [RFC6890]  Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman,
              "Special-Purpose IP Address Registries", BCP 153,
              RFC 6890, DOI 10.17487/RFC6890, April 2013,

   [RFC7453]  Mahalingam, V., Sampath, K., Aldrin, S., and T. Nadeau,
              "MPLS Transport Profile (MPLS-TP) Traffic Engineering (TE)
              Management Information Base (MIB)", RFC 7453,
              DOI 10.17487/RFC7453, February 2015,

   [RFC8932]  Dickinson, S., Overeinder, B., van Rijswijk-Deij, R., and
              A. Mankin, "Recommendations for DNS Privacy Service
              Operators", BCP 232, RFC 8932, DOI 10.17487/RFC8932,
              October 2020, <>.

12.  Informative References

   [Atlas]    RIPE Network Coordination Centre, "RIPE Atlas",

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              Strong, M., "Fixing reachability to, GLOBALLY!", 4
              April 2018, <

   [Huston]   Huston, G., "Detecting IP Address Filters", 13 January
              2012, <

              NLNOG RING, "10 Years of NLNOG RING",

              Aben, E., "The Curious Case of 128.0/16", 6 December 2011,

              Schwarzinger, F., "Pollution in 1/8", 3 February 2010,

              Aben, E., "The Debogonisation of 2a10::/12", 17 January
              2020, <

Appendix A.  Implementation Status

   The behavior specified by this document has been implemented by the
   Linux kernel since version 5.2, released in July 2019.  Accordingly,
   it has been included in various operating system releases, including
   Ubuntu 19.10 and Fedora 31 from October 2019, and some Android 11 and
   12 devices.

   This behavior has also been implemented by the OpenBSD kernel and
   userspace since May 6, 2022, and hence appears in OpenBSD 7.2,
   released on October 20, 2022.

   This behavior is disabled by default in FreeBSD, but enabled by
   "sysctl net.inet.ip.allow_net0=1", available in FreeBSD 14.0,
   released in November 2023.  It has been available in development
   releases since July 13, 2022.

   We have prepared a patch which enables this behavior on NetBSD.  It
   has not been merged as of December 2023.

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   Routing of subnets in the 0/8 range is supported by the Gobgp routing
   daemon, as of release 3.0.0 in March 2022 (or earlier).

   Support for 0/8 addressing may be typical of many DHCP
   implementations (because the 0/8 address assignment special case has
   often been handled at the kernel level).  If the underlying operating
   system supports 0/8 assignment to an interface, the final official
   ISC DHCP release (4.4.3) supports 0/8 allocation as both 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

   John Gilmore
   IPv4 Unicast Extensions Project
   PO Box 170640-rfc
   San Francisco, CA 94117-0640
   United States of America

   David M. Täht
   IPv4 Unicast Extensions Project
   Half Moon Bay, CA
   United States of America

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