Network Working Group                                        Y. Rekhter
                                                          Cisco Systems
                                                           B. Moskowitz
                                                         Chrysler Corp.
                                                          D. Karrenberg
                                                               RIPE NCC
                                                         G. J. de Groot
                                                               RIPE NCC
                                                              July 1995

                Address Allocation for Private Internets

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   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.''

   To learn the current status of any Internet-Draft, please check the
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1. Introduction

   This document describes address allocation for private internets. The
   allocation permits full network layer connectivity between all hosts
   inside an enterprise as well as between all public hosts on different

   For the purposes of this memo, an enterprise is an entity
   autonomously operating a network using TCP/IP and in particular
   determining the addressing plan and address assignments within that

2. Motivation

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   With the proliferation of TCP/IP technology worldwide, including
   outside the Internet itself, an increasing number of non-connected
   enterprises use this technology and its addressing capabilities for
   sole intra-enterprise communications, without any intention to ever
   directly connect to other enterprises or the Internet itself.

   The current practice is to assign globally unique addresses to all
   hosts that use TCP/IP.  There is a growing concern that the finite IP
   address space might become exhausted.  Therefore, the guidelines for
   assigning IP address space have been tightened in recent years [1].
   These rules are often more conservative than enterprises would like,
   in order to implement and operate their networks.

   Acquiring globally unique addresses from an Internet registry is no
   longer sufficient to achieve Internet-wide IP connectivity. In the
   past assignment of globally unique addresses had been sufficient to
   insure Internet-wide reachability to these addresses.  With the
   current size of the Internet and its growth rate this is no longer
   true -- it is no longer realistic to assume that just by virtue of
   acquiring globally unique IP addresses out of an Internet registry an
   organization that acquires such addresses would have Internet-wide IP
   connectivity once the organization gets connected to the Internet.
   To the contrary, it is quite likely that when the organization would
   connect to the Internet to achieve Internet-wide IP connectivity the
   organization would need to change IP addresses (renumber) all of its
   public hosts, regardless of whether the addresses used by the
   organization initially were globally unique or not.

   Hosts within enterprises that use IP can be partitioned into three

      - hosts that do not require access to hosts in other enterprises
        or the Internet at large;

      - hosts that need access to a limited set of outside services
        (e.g., E-mail, FTP, netnews, remote login) which can be handled
        by application layer gateways;

      - hosts that need network layer access outside the enterprise
        (provided via IP connectivity);

      - hosts within the first category may use IP addresses that are
        unambiguous within an enterprise, but may be ambiguous between

   For many hosts in the second category an unrestricted external access
   (provided via IP connectivity) may be unnecessary and even
   undesirable for privacy/security reasons.  Just like hosts within the

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   first category, such hosts may use IP addresses that are unambiguous
   within an enterprise, but may be ambiguous between enterprises.

   Only hosts in the last category require IP addresses that are
   globally unambiguous.

   Many applications require connectivity only within one enterprise and
   do not even need external connectivity for the majority of internal
   hosts.  In larger enterprises it is often easy to identify a
   substantial number of hosts using TCP/IP that do not need network
   layer connectivity outside the enterprise.

   Some examples, where external connectivity might not be required,

      - A large airport which has its arrival/departure displays
        individually addressable via TCP/IP. It is very unlikely that
        these displays need to be directly accessible from other

      - Large organisations like banks and retail chains are switching
        to TCP/IP for their internal communication.  Large numbers of
        local workstations like cash registers, money machines, and
        equipment at clerical positions rarely need to have such

      - For security reasons, many enterprises use application layer
        gateways (e.g., firewalls) to connect their internal network to
        the Internet.  The internal network usually does not have direct
        access to the Internet, thus only one or more firewall hosts are
        visible from the Internet.  In this case, the internal network
        can use non-unique IP numbers.

      - If two enterprises communicate over their own private link,
        usually only a very limited set of hosts is mutually reachable
        from the other enterprise over this link. Only those hosts need
        globally unique IP numbers.

      - Interfaces of routers on an internal network usually do not
        need to be directly accessible from outside the enterprise.

3. Private Address Space

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   The Internet Assigned Numbers Authority (IANA) has reserved the
   following three blocks of the IP address space for private networks:        -  (10/8 prefix)      -  (172.16/12 prefix)     - (192.168/16 prefix)

   We will refer to the first block as "24-bit block", the second as
   "20-bit block, and to the third as "16-bit" block.  Note that the
   first block is nothing but a single class A network number, while the
   second block is a set of 16 contiguous class B network numbers, and
   third block is a set of 256 contiguous class C network numbers.

   An enterprise that decides to use IP addresses out of the address
   space defined in this document can do so without any coordination
   with IANA or an Internet registry.  The address space can thus be
   used by many enterprises.  Addresses within this private address
   space will only be unique within the enterprise, or the set of
   enterprises which choose to cooperate over this space so they may
   communicate with eachother in their own private internet.

   As before, any enterprise that needs globally unique address space is
   required to obtain such addresses from an Internet registry.  An
   enterprise that requests IP addresses for its external connectivity
   will never be assigned addresses from the blocks defined above.

   In order to use private address space, an enterprise needs to
   determine which hosts do not need to have network layer connectivity
   outside the enterprise in the foreseeable future.  Such hosts will be
   called private hosts, and will use the private address space defined
   above.  Private hosts can communicate with all other hosts inside the
   enterprise, both public and private.  However, they cannot have IP
   connectivity to any external host.  While not having external network
   layer connectivity private hosts can still have access to external
   services via application layer relays.

   All other hosts will be called public and will use globally unique
   address space assigned by an Internet Registry.  Public hosts can
   communicate with other hosts inside the enterprise both public and
   private and can have IP connectivity to external public hosts.
   Public hosts do not have connectivity to private hosts of other

   Moving a host from private to public or vice versa involves a change
   of IP address.

   Because private addresses have no global meaning, routing information
   about private networks shall not be propagated on inter-enterprise

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   links, and packets with private source or destination addresses
   should not be forwarded across such links.  Routers in networks not
   using private address space, especially those of Internet service
   providers, are expected to be configured to reject (filter out)
   routing information about private networks.  If such a router
   receives such information the rejection shall not be treated as a
   routing protocol error.

   Indirect references to such addresses should be contained within the
   enterprise.  Prominent examples of such references are DNS Resource
   Records and other information referring to internal private
   addresses.  In particular, Internet service providers should take
   measures to prevent such leakage.

4. Advantages and Disadvantages of Using Private Address Space

   The obvious advantage of using private address space for the Internet
   at large is to conserve the globally unique address space by not
   using it where global uniqueness is not required.

   Enterprises themselves also enjoy a number of benefits from their
   usage of private address space: They gain a lot of flexibility in
   network design by having more address space at their disposal than
   they could obtain from the globally unique pool.  This enables
   operationally and administratively convenient addressing schemes as
   well as easier growth paths.

   For a variety of reasons the Internet has already encountered
   situations where an enterprise that has not been connected to the
   Internet had used IP address space for its hosts without getting this
   space assigned from the IANA.  In some cases this address space had
   been already assigned to other enterprises.  When such an enterprise
   later connects to the Internet, it could potentially create very
   serious problems, as IP routing cannot provide correct operations in
   presence of ambiguous addressing.  Using private address space
   provides a safe choice for such enterprises, avoiding clashes once
   outside connectivity is needed.

   One could argue that the potential need for renumbering represents a
   significant drawback of using the addresses out of the block
   allocated for private internets.  However, we need to observe that
   the need is only "potential", since many hosts may never move into
   the third category, and an enterprise may never decide to
   interconnect (at IP level) with another enterprise.

   But even if renumbering has to happen, we have to observe that with
   Classless Inter-Domain Routing (CIDR) an enterprise that is connected
   to the Internet may be encouraged (or even required) to renumber its

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   public hosts, as it changes its Network Service Providers. As the
   Internet grows, to contain the routing information overhead in the
   Internet the renumbering is likely to happen more often in the
   future, regardless of whether an enterprise does or does not use the
   addresses out of the block allocated for private networks. Tools to
   facilitate renumbering (e.g., DHCP) would certainly make it less of a

   Also observe that the clear division of public and private hosts and
   the resulting need to renumber makes uncontrolled outside
   connectivity more difficult, so to some extend the need to renumber
   could be viewed as an advantage.

5. Operational Considerations

   A recommended strategy is to design the private part of the network
   first and use private address space for all internal links.  Then
   plan public subnets at the locations needed and design the external

   This design is not fixed permanently.  If a number of hosts require
   to change status later this can be accomplished by renumbering only
   the hosts involved and installing another physical subnet if

   If a suitable subnetting scheme can be designed and is supported by
   the equipment concerned, it is advisable to use the 24-bit block of
   private address space and make an addressing plan with a good growth
   path.  If subnetting is a problem, the 16-bit class C block, which
   consists of 255 contiguous class C network numbers, can be used.

   Using multiple IP (sub)nets on the same physical medium has many
   pitfalls. We recommend to avoid it unless the operational problems
   are well understood and it is proven that all equipment supports this

   Moving a single host between private and public status will involve a
   change of address and in most cases physical connectivity.  In
   locations where such changes can be foreseen (machine rooms etc.)  it
   may be advisable to configure separate physical media for public and
   private subnets to facilitate such changes.

   Changing the status of all hosts on a whole (sub)network can be done
   easily and without disruption for the enterprise network as a whole.
   Consequently it is advisable to group hosts whose connectivity needs
   might undergo similar changes in the future on their own subnets.

   It is strongly recommended that routers which connect enterprises to

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   external networks are set up with appropriate packet and routing
   filters at both ends of the link in order to prevent packet and
   routing information leakage.  An enterprise should also filter any
   private networks from inbound routing information in order to protect
   itself from ambiguous routing situations which can occur if routes to
   the private address space point outside the enterprise.

   Groups of organisations which foresee a big need for mutual
   communication can consider forming an enterprise by designing a
   common addressing plan supported by the necessary organisational
   arrangements like a registry.

   If two (or more) organizations follow the address allocation
   specified in this document and then later wish to establish IP
   connectivity with each other, then there is a risk that address
   uniqueness would be violated. To minimize the risk it is strongly
   recommended that an organization that decides to use addresses out of
   the blocks specified in this document selects a random contiguous
   sub-block(s) for its internal allocation.

   If two sites of the same enterprise need to be connected using an
   external service provider, they can consider using an IP tunnel to
   prevent packet leaks from the private network.

   A possible approach to avoid leaking of DNS RRs is to run two
   nameservers, one external server authoritative for all globally
   unique IP addresses of the enterprise and one internal nameserver
   authoritative for all IP addresses of the enterprise, both public and
   private.  In order to ensure consistency both these servers should be
   configured from the same data of which the external nameserver only
   receives a filtered version.

   The resolvers on all internal hosts, both public and private, query
   only the internal nameserver.  The external server resolves queries
   from resolvers outside the enterprise and is linked into the global
   DNS.  The internal server forwards all queries for information
   outside the enterprise to the external nameserver, so all internal
   hosts can access the global DNS.  This ensures that information about
   private hosts does not reach resolvers and nameservers outside the

6. References

   [1] Gerich, E., "Guidelines for Management of IP Address Space", RFC
       1466, Merit Network, Inc., May 1993.

7. Security Considerations

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   While using private address space can improve security, it is not a
   substitute for dedicated security measures.

8. Conclusion

   With the described scheme many large enterprises will need only a
   relatively small block of addresses from the globally unique IP
   address space.  The Internet at large benefits through conservation
   of globally unique address space which will effectively lengthen the
   lifetime of the IP address space. The enterprises benefit from the
   increased flexibility provided by a relatively large private address

9. Acknowledgments

   We would like to thank Tony Bates (RIPE NCC), Jordan Becker (ANS),
   Hans-Werner Braun (SDSC), Ross Callon (Wellfleet), John Curran
   (NEARNET), Vince Fuller (Barrnet), Tony Li (cisco Systems), Anne Lord
   (RIPE NCC), Milo Medin (NSI), Marten Terpstra (RIPE NCC), Geza
   Turchanyi (RIPE NCC), Christophe Wolfhugel (Pasteur Institute), Andy
   Linton (, Brian Carpenter (CERN), and Randy Bush (PSG)
   for their review and constructive comments.

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10. Authors' Addresses

   Yakov Rekhter
   Cisco systems
   170 West Tasman Drive
   San Jose, CA, USA

   Phone: +1 914 528 0090
   Fax: +1 914 XXX XXXX

   Robert G Moskowitz
   Chrysler Corporation
   CIMS: 424-73-00
   25999 Lawrence Ave
   Center Line, MI 48015

   Phone: +1 810 758 8212
   Fax: +1 810 758 8173

   Daniel Karrenberg
   RIPE Network Coordination Centre
   Kruislaan 409
   1098 SJ Amsterdam, the Netherlands

   Phone: +31 20 592 5065
   Fax: +31 20 592 5090

   Geert Jan de Groot
   RIPE Network Coordination Centre
   Kruislaan 409
   1098 SJ Amsterdam, the Netherlands

   Phone: +31 20 592 5065
   Fax: +31 20 592 5090

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