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.
                                                              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.

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   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 [CITE growth
   ietf proceedings)].  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
   progress within the community to find long term solutions to both
   these problems.  Meanwhile it is necessary to revisit  address
   allocation procedures, and their impact on the Internet routing

   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 appear to other providers as a single route [RFC1518],

   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.

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   The current practice is to assign globally unique addresses to all
   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 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 (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.

         - 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
   application layer relays.

   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.  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.

   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.  Although not the case today, it is likely
   that renumbering may be necessary, regardless of whether private or
   public address space is used.

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

   If we review the examples we list in Section 2, we note that banks
   tend to merge, as do companies with firewalls.  Therefore it may be
   advisable to discuss with management the implications of private

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

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   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 block of 255 /24 prefixes 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.

   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
   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. References

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



7. Security Considerations

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

9. Acknowledgments

   We would like to thank Tony Bates (RIPE NCC), Jordan Becker (ANS),
   Hans-Werner Braun (SDSC), Ross Callon (BayNetworks), John Curran
   (NEARNET), Vince Fuller (Barrnet), Tony Li (cisco Systems), Anne Lord

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   (RIPE NCC), Milo Medin (NSI), Marten Terpstra (RIPE NCC), Geza
   Turchanyi (RIPE NCC), Christophe Wolfhugel (Pasteur Institute), Andy
   Linton (, Brian Carpenter (CERN), Randy Bush (PSG),
   Erik Fair (Apple Computer), Dave Crocker (XXX), Tom Kessler (SGI),
   and Dave Piscitello (Core Competence) 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: +1 408 526-4952

   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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