TitleAddress Allocation for Private Internets
AuthorY. Rekhter, B. Moskowitz, D. Karrenberg, G. de Groot
DateMarch 1994
Format:TXT, HTML
Obsoleted byRFC1918

Network Working Group                                        Y. Rekhter
Request for Comments: 1597       T.J. Watson Research Center, IBM Corp.
Category: Informational                                    B. Moskowitz
                                                         Chrysler Corp.
                                                          D. Karrenberg
                                                               RIPE NCC
                                                            G. de Groot
                                                               RIPE NCC
                                                             March 1994

                Address Allocation for Private Internets

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

1. Introduction

   This RFC describes methods to preserve IP address space by not
   allocating globally unique IP addresses to hosts private to an
   enterprise while still permitting full network layer connectivity
   between all hosts inside an enterprise as well as between all public
   hosts of different enterprises. The authors hope, that using these
   methods, significant savings can be made on allocating IP address

   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

   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.

RFC 1597        Address Allocation for Private Internets      March 1994

   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
   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

RFC 1597        Address Allocation for Private Internets      March 1994

      - 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

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

   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 255 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.

   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

RFC 1597        Address Allocation for Private Internets      March 1994

   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
   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 between 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

RFC 1597        Address Allocation for Private Internets      March 1994

   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 to renumber its public hosts, as it
   changes its Network Service Providers.  Thus 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 concern.

   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

RFC 1597        Address Allocation for Private Internets      March 1994

   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.

   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 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 form 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

RFC 1597        Address Allocation for Private Internets      March 1994

6. References

   [1] 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

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), and Geza
   Turchanyi (RIPE NCC) for their review and constructive comments.

RFC 1597        Address Allocation for Private Internets      March 1994

10. Authors' Addresses

   Yakov Rekhter
   T.J. Watson Research Center, IBM Corp.
   P.O. Box 218
   Yorktown Heights, NY, 10598

   Phone: +1 914 945 3896
   Fax: +1 914 945 2141
   EMail: yakov@watson.ibm.com

   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
   EMail: 3858921@mcimail.com

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

   Phone: +31 20 592 5065
   Fax: +31 20 592 5090
   EMail: Daniel.Karrenberg@ripe.net

   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
   EMail: GeertJan.deGroot@ripe.net