rfc1597.txt

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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
   space.

   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
   network.

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.



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RFC 1597        Address Allocation for Private Internets      March 1994


   Hosts within enterprises that use IP can be partitioned into three
   categories:

      - 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
        enterprises.

   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,
   are:

      - 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
         networks.

      - 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
        connectivity.






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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:

        10.0.0.0        -   10.255.255.255
        172.16.0.0      -   172.31.255.255
        192.168.0.0     -   192.168.255.255

   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



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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
   enterprises.

   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



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