    Network Working Group                Internet Engineering Steering Group
    Request for Comments: 1517                             R. Hinden, Editor
    Category: Standards Track                                 September 1993

               Applicability Statement for the Implementation of
                     Classless Inter-Domain Routing (CIDR)

    Status of this Memo

       This RFC specifies an Internet standards track protocol for the
       Internet community, and requests discussion and suggestions for
       improvements.  Please refer to the current edition of the "Internet
       Official Protocol Standards" for the standardization state and status
       of this protocol.  Distribution of this memo is unlimited.

    1.   Introduction

       As the Internet has evolved and grown in recent years, it has become
       clear that it will soon face several serious scaling problems. These
       include:

          - Exhaustion of the class-B network address space. One
            fundamental cause of this problem is the lack of a network
            class of a size that is appropriate for a mid-sized
            organization. Class-C, with a maximum of 254 host addresses, is
            too small, while class-B, which allows up to 65534 addresses,
            is too large to be densely populated.  The result is inefficient
            utilization of class-B network numbers.

          - Routing information overload. The size and rate of growth of the
            routing tables in Internet routers is beyond the ability of
            current software (and people) to effectively manage.

          - Eventual exhaustion of IP network numbers.

       It has become clear that the first two of these problems are likely
       to become critical in the near term.  Classless Inter-Domain Routing
       (CIDR) ttempts to deal with these problems by defining a mechanism to
       slow the growth of routing tables and reduce the need to allocate new
       IP network numbers.  It does not attempt to solve the third problem,
       which is of a more long-term nature, but instead endeavors to ease
       enough of the short to mid-term difficulties to allow the Internet to
       continue to function efficiently while progress is made on a longer-
       term solution.

       The IESG, after a thorough discussion in the IETF, in June 1992
       selected CIDR as the solution for the short term routing table

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    RFC 1517              CIDR Applicability Statement        September 1993

       explosion problem [1].

    2. Components of the Architecture

       The CIDR architecture is described in the following documents:

          - "An Architecture for IP Address Allocation with CIDR" [2]

          - "Classless Inter-Domain Routing (CIDR):  An Address Assignment
            and Aggregation Strategy" [3]

       The first of these documents presents the overall architecture of
       CIDR; the second describes the specific address allocation scheme to
       be used.

       In addition to these two documents, "Guidelines for Management of IP
       Address Space" [4] provides specific recommendations for assigning IP
       addresses that are consistent with [2] and [3], and "Status of CIDR
       Deployment in the Internet" [5] describes the timetable for deploying
       [4] in the Internet.  Both [4] and [5] should be viewed as
       supporting, rather than defining, documents.

       In addition to the documents mentioned above, CIDR requires that
       inter-domain routing protocols be capable of handling reachability
       information that is expressed solely in terms of IP address prefixes.
       While several inter-domain routing protocols are capable of
       supporting such functionality, this Applicability Statement does not
       mandate the use of a particular one.

       Although Internet routing domains are not required to use routing
       protocols capable of propagating CIDR routes, the topology such
       routing domains can support will be somewhat limited.  In particular,
       the non-CIDR-capable parts of the Internet will need to default
       towards the CIDR-capable parts of the Internet for routes which have
       been aggregated to non-network boundaries.

    3. Applicability of CIDR

       The CIDR architecture is applicable to any group of connected domains
       that supports IP version 4 [6] [7].  CIDR does not require all of the
       domains in the Internet to be converted to use CIDR. It assumes that
       some of the existing domains in the Internet will never be able to
       convert.  Despite this, CIDR will still provide connectivity to such
       places, although the optimality of routes to these places may be
       impacted.

       This Applicability Statement requires Internet domains providing
       backbone and/or transit service to fully implement CIDR in order to

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    RFC 1517              CIDR Applicability Statement        September 1993

       ensure that the growth of the resources required by routers to
       provide Internet-wide connectivity will be significantly slower than
       the growth of the number of assigned networks.

       This Applicability Statement strongly recommends that all non-
       backbone/transit Internet domains also implement CIDR because it will
       reduce the amount of routing information inside of these domains.

       Individual domains are free to choose whatever inter-domain and
       intra-domain routing architectures best meet their requirements.
       Specifically, this Applicability Statement does not prevent a domain
       or a group of domains from using addressing schemes which do not
       conform to CIDR.  Subject to the available resources in routers, CIDR
       should be able to co-exist with other addressing schemes without
       adversely impacting overall connectivity.

    4. References

       [1] Gross, P., and P. Almquist, "IESG Deliberations on Routing and
           Addressing", RFC 1380, IESG Chair, IESG Internet AD, November
           1992.

       [2] Rekhter, Y., and T. Li, "An Architecture for IP Address
           Allocation with CIDR", RFC 1518, T.J. Watson Research Center, IBM
           Corp., cisco Systems, September 1993.

       [3] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless Inter-
           Domain Routing (CIDR): An Address Assignment and Aggregation
           Strategy", RFC 1519, BARRNet, cisco, Merit, and OARnet, September
           1993.

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

       [5] Topolcic, C., "Status of CIDR Deployment in the Internet", RFC
           1467, CNRI, August 1993.

       [6] Postel, J., "Internet Protocol - DARPA Internet Program Protocol
           Specification", STD 5, RFC 791, USC/Information Sciences
           Institute, September 1981.

       [7] Braden, R., Editor, "Requirements for Internet Hosts --
           Communication Layers", STD 3, RFC 1122, IETF, October 1989.

    5. Security Considerations

       Security issues are not discussed in this memo.

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    RFC 1517              CIDR Applicability Statement        September 1993

    6. Author's Address

       Robert M. Hinden
       Sun Microsystems
       2550 Garcia Ave, MS MTV5-44
       Mt. View, CA 94043

       Phone: (415) 336-2082
       Fax:   (415) 336-6015

       EMail: hinden@eng.sun.com

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