Network Working Group                                        Y. Rekhter
                                                          Cisco Systems
                                                           B. Moskowitz
                                                         Chrysler Corp.
                                                          D. Karrenberg
                                                               RIPE NCC
                                                         G. J. de Groot
                                                               RIPE NCC
                                                                E. Lear
                                                 Silicon Graphics, Inc.
                                                         September 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
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   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

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

   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 of different
   enterprises. The cost of using private internet address space is the
   potentially costly effort to renumber hosts and networks between

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   public and private.

2. Motivation

   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 Internet has grown beyond anyone's expectations. Sustained
   exponential growth continues to introduce new challenges.  One
   challenge is a concern within the community that globally unique
   address space will be exhausted. A separate and far more pressing
   concern is that the amount of routing overhead will grow beyond the
   capabilities of Internet Service Providers. Efforts are in progress
   within the community to find long term solutions to both of these
   problems. Meanwhile it is necessary to revisit address allocation
   procedures, and their impact on the Internet routing system.

   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. To contain
   growth of routing overhead, an Internet Provider obtains a block of
   address space from an address registry, and then assigns to its
   customers addresses from within that block based on each customer
   requirement. The result of this process is that routes to many
   customers will be aggregated together, and will appear to other
   providers as a single route [RFC1518], [RFC1519].

   In order for route aggregation to be effective, Internet providers
   encourage customers joining their network to use the provider's
   block, and thus renumber their computers. Such encouragement may
   become a requirement in the future. With the current size of the
   Internet and its growth rate it is no longer realistic to assume that
   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 (hosts that require
   Internet-wide IP connectivity), regardless of whether the addresses
   used by the organization initially were globally unique or not.

   The current practice is to assign globally unique addresses to all

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   hosts that use TCP/IP. In order to extend the life of the IPv4
   address space, address registries are requiring more justification
   than ever before, making it harder for organizations to acquire
   additional address space [RFC1466].

   Hosts within enterprises that use IP can be partitioned into three

      Category 1: hosts that do not require access to hosts in other
                  enterprises or the Internet at large; hosts within
                  this category may use IP addresses that are unambiguous
                  within an enterprise, but may be ambiguous between

      Category 2: hosts that need access to a limited set of outside
                  services (e.g., E-mail, FTP, netnews, remote login)
                  which can be handled by mediating gateways (e.g.
                  application layer gateways). For many hosts in this
                  category an unrestricted external access (provided
                  via IP connectivity) may be unnecessary and even
                  undesirable for privacy/security reasons. Just like
                  hosts within the first category, such hosts may use
                  IP addresses that are unambiguous within an enterprise,
                  but may be ambiguous between enterprises.

      Category 3: hosts that need network layer access outside the
                  enterprise (provided via IP connectivity); hosts in
                  the last category require IP addresses that are globally

   We will refer to the hosts in the first and second categories as
   "private".  We will refer to the hosts in the third category as

   Many applications require connectivity only within one enterprise and
   do not need external (outside the enterprise) 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

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           these displays need to be directly accessible from other

         - Large organizations 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 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 gateways are visible from
           the Internet. In this case, the internal network can use
           non-unique IP network numbers.

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

3. Private Address Space

   The Internet Assigned Numbers Authority (IANA) has reserved the
   following three blocks of the IP address space for private internets:        -  (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 each
   other 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

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   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 and thus could be
   classified as private. Such hosts 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 host outside of the enterprise. While not
   having external (outside of the enterprise) IP connectivity private
   hosts can still have access to external services via mediating
   gateways (e.g.  application layer gateways).

   All other hosts will be 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 public hosts outside the enterprise.
   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
   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

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   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. If such an enterprise
   would later connects to the Internet, this could potentially create
   very serious problems, as IP routing cannot provide correct
   operations in presence of ambiguous addressing. Although in principle
   Internet Service Providers should guard against such mistakes through
   the use of route filters, this not always happen in practice. Using
   private address space provides a safe choice for such enterprises,
   avoiding clashes once outside connectivity is needed.

   A major drawback to the use of private address space is that it may
   actually reduce an enterprise's flexibility to access the Internet.
   Once one commits to using a private address, one is committing to
   renumber all or part of an enterprise, should one decide to route an
   entire enterprise to the Internet. Usually the cost of renumbering
   can be measured by counting the number of hosts that have to
   transition from private to public. As was discussed earlier, however,
   even if a network uses globally unique addresses, it may still have
   to renumber in order to acquire Internet-wide IP connectivity.

   Another drawback to the use of private address space is that it may
   require renumbering when merging several private internets into a
   single private internet. If we review the examples we list in Section
   2, we note that companies tend to merge. If such companies prior to
   the merge maintained their uncoordinated internets using private
   address space, then if after the merge these private internets would
   be combined into a single private internet, some addresses within the
   combined private internet may not be unique. As a result, hosts with
   these addresses would need to be renumbered.

   The cost of renumbering may well be mitigated by development and
   deployment of tools that facilitate renumbering (e.g. Dynamic Host
   Configuration Protocol (DHCP)). When deciding whether to use private
   addresses, we recommend to inquire computer and software vendors
   about availability of such tools.

5. Operational Considerations

   One possible strategy is to design the private part of the network
   first and use private address space for all internal links. Then plan

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   public subnets at the locations needed and design the external

   This design does not need to be fixed permanently. If a group of one
   or more hosts requires to change their status (from private to public
   or vice versa) later, this can be accomplished by renumbering only
   the hosts involved, and changing physical connectivity, if needed. In
   locations where such changes can be foreseen (machine rooms, etc.),
   it is advisable to configure separate physical media for public and
   private subnets to facilitate such changes.  In order to avoid major
   network disruptions, it is advisable to group hosts with similar
   connectivity needs on their own subnets.

   If a suitable subnetting scheme can be designed and is supported by
   the equipment concerned, it is advisable to use the 24-bit block
   (class A network) of private address space and make an addressing
   plan with a good growth path. If subnetting is a problem, the 16-bit
   block (class C networks), or the 20-bit block (class B networks) of
   private address space can be used.

   One might be tempted to have both public and private addresses on the
   same physical medium. While this is possible, there are pitfalls to
   such a design. We advise caution when proceeding in this area.

   It is strongly recommended that routers which connect enterprises to
   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.

   It is possible for two sites, who both coordinate their private
   address space, to communicate with each other over a public network.
   To do so they must use some method of encapsulation at their borders
   to a public network, thus keeping their private addresses private.

   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.

   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

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   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. Security Considerations

   Security Considerations are not addressed in this document.

7. 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
   space. However, use of private addressing requires that an
   organization renumber part or all of its enterprise network, as its
   connectivity requirements change over time.

8. Acknowledgments

   We would like to thank Tony Bates (MCI), Jordan Becker (ANS), Hans-
   Werner Braun (SDSC), Ross Callon (BayNetworks), John Curran
   (NEARNET), Vince Fuller (Barrnet), Tony Li (cisco Systems), Anne Lord
   (RIPE NCC), Milo Medin (NSI), Marten Terpstra (BayNetworks), Geza
   Turchanyi (RIPE NCC), Christophe Wolfhugel (Pasteur Institute), Andy
   Linton (, Brian Carpenter (CERN), Randy Bush (PSG),
   Erik Fair (Apple Computer), Dave Crocker (Brandenburg Consulting),
   Tom Kessler (SGI), and Dave Piscitello (Core Competence) for their
   review and constructive comments.

9. References

   [RFC1466] Gerich, E., "Guidelines for Management of IP Address
       Space", RFC 1466, Merit Network, Inc., May 1993.

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   [RFC1518] Rekhter, Y., Li, T., "An Architecture for IP Address
       Allocation with CIDR", September 1993

   [RFC1519] Fuller, V., Li, T., Yu, J., Varadhan, K., "Classless
       Inter-Domain Routing (CIDR): an Address Assignment and
       Aggregation Strategy", September 1993

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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: +1 408 526-4952

       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

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       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

       Eliot Lear
       Mail Stop 15-730
       Silicon Graphics, Inc.
       2011 N. Shoreline Blvd.
       Mountain View, CA 94043-1389

       Phone: +1 415 960 1980
       Fax:   +1 415 961 9584

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