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rfc:rfc1918

Network Working Group Y. Rekhter Request for Comments: 1918 Cisco Systems Obsoletes: 1627, 1597 B. Moskowitz BCP: 5 Chrysler Corp. Category: Best Current Practice D. Karrenberg

                                                              RIPE NCC
                                                        G. J. de Groot
                                                              RIPE NCC
                                                               E. Lear
                                                Silicon Graphics, Inc.
                                                         February 1996
              Address Allocation for Private Internets

Status of this Memo

 This document specifies an Internet Best Current Practices for the
 Internet Community, and requests discussion and suggestions for
 improvements.  Distribution of this memo is unlimited.

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

Rekhter, et al Best Current Practice [Page 1] RFC 1918 Address Allocation for Private Internets February 1996

 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.
 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.
 It has been typical 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
 categories:
    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 enterprises.
    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

Rekhter, et al Best Current Practice [Page 2] RFC 1918 Address Allocation for Private Internets February 1996

                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 unambiguous.
 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
 "public".
 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,
 are:
  1. 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.
  1. 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 connectivity.
  1. 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.
  1. Interfaces of routers on an internal network usually do not

need to be directly accessible from outside the enterprise.

Rekhter, et al Best Current Practice [Page 3] RFC 1918 Address Allocation for Private Internets February 1996

3. Private Address Space

 The Internet Assigned Numbers Authority (IANA) has reserved the
 following three blocks of the IP address space for private internets:
   10.0.0.0        -   10.255.255.255  (10/8 prefix)
   172.16.0.0      -   172.31.255.255  (172.16/12 prefix)
   192.168.0.0     -   192.168.255.255 (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 (in
 pre-CIDR notation) 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
 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
 enterprises.

Rekhter, et al Best Current Practice [Page 4] RFC 1918 Address Allocation for Private Internets February 1996

 Moving a host from private to public or vice versa involves a change
 of IP address, changes to the appropriate DNS entries, and changes to
 configuration files on other hosts that reference the host by 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 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 does not always happen in practice.
 Using private address space provides a safe choice for such
 enterprises, avoiding clashes once outside connectivity is needed.

Rekhter, et al Best Current Practice [Page 5] RFC 1918 Address Allocation for Private Internets February 1996

 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 part or all of an enterprise, should one decide to provide
 IP connectivity between that part (or all of the enterprise) and 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.  A separate IETF effort (PIER
 Working Group) is pursuing full documentation of the requirements and
 procedures for renumbering.

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
 public subnets at the locations needed and design the external
 connectivity.
 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.

Rekhter, et al Best Current Practice [Page 6] RFC 1918 Address Allocation for Private Internets February 1996

 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 (note that the pitfalls have nothing to do with the use
 of private addresses, but are due to the presence of multiple IP
 subnets on a common Data Link subnetwork).  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 using private IP addresses choose
 randomly from the reserved pool of private addresses, when allocating
 sub-blocks for its internal allocation.
 If an enterprise uses the private address space, or a mix of private
 and public address spaces, then DNS clients outside of the enterprise
 should not see addresses in the private address space used by the
 enterprise, since these addresses would be ambiguous.  One way to
 ensure this is to run two authority servers for each DNS zone
 containing both publically and privately addressed hosts.  One server
 would be visible from the public address space and would contain only
 the subset of the enterprise's addresses which were reachable using
 public addresses.  The other server would be reachable only from the
 private network and would contain the full set of data, including the
 private addresses and whatever public addresses are reachable the
 private network.  In order to ensure consistency, both servers should
 be configured from the same data of which the publically visible zone

Rekhter, et al Best Current Practice [Page 7] RFC 1918 Address Allocation for Private Internets February 1996

 only contains a filtered version. There is certain degree of
 additional complexity associated with providing these capabilities.

6. Security Considerations

 Security issues are not addressed in this memo.

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 (BBN
 Planet), Vince Fuller (BBN Planet), Tony Li (cisco Systems), Anne
 Lord (RIPE NCC), Milo Medin (NSI), Marten Terpstra (BayNetworks),
 Geza Turchanyi (RIPE NCC), Christophe Wolfhugel (Pasteur Institute),
 Andy Linton (connect.com.au), Brian Carpenter (CERN), Randy Bush
 (PSG), Erik Fair (Apple Computer), Dave Crocker (Brandenburg
 Consulting), Tom Kessler (SGI), Dave Piscitello (Core Competence),
 Matt Crawford (FNAL), Michael Patton (BBN), and Paul Vixie (Internet
 Software Consortium) 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.
 [RFC1518] Rekhter, Y., and T. Li, "An Architecture for IP Address
     Allocation with CIDR", RFC 1518, September 1993.
 [RFC1519] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless
     Inter-Domain Routing (CIDR): an Address Assignment and
     Aggregation Strategy", RFC 1519, September 1993.

Rekhter, et al Best Current Practice [Page 8] RFC 1918 Address Allocation for Private Internets February 1996

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
 EMail: yakov@cisco.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: rgm3@is.chrysler.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
 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
 EMail: lear@sgi.com

Rekhter, et al Best Current Practice [Page 9]

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