An Architecture for IP Address Allocation with CIDR
RFC 1518

Document Type RFC - Historic (September 1993; No errata)
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Network Working Group                                         Y. Rekhter
Request for Comments: 1518        T.J. Watson Research Center, IBM Corp.
Category: Standards Track                                          T. Li
                                                           cisco Systems
                                                                 Editors
                                                          September 1993

          An Architecture for IP Address Allocation with 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

   This paper provides an architecture and a plan for allocating IP
   addresses in the Internet. This architecture and the plan are
   intended to play an important role in steering the Internet towards
   the Address Assignment and Aggregating Strategy outlined in [1].

   The IP address space is a scarce shared resource that must be managed
   for the good of the community. The managers of this resource are
   acting as its custodians. They have a responsibility to the community
   to manage it for the common good.

2.  Scope

   The global Internet can be modeled as a collection of hosts
   interconnected via transmission and switching facilities.  Control
   over the collection of hosts and the transmission and switching
   facilities that compose the networking resources of the global
   Internet is not homogeneous, but is distributed among multiple
   administrative authorities. Resources under control of a single
   administration form a domain.  For the rest of this paper, "domain"
   and "routing domain" will be used interchangeably.  Domains that
   share their resources with other domains are called network service
   providers (or just providers). Domains that utilize other domain's
   resources are called network service subscribers (or just
   subscribers).  A given domain may act as a provider and a subscriber
   simultaneously.

Rekhter & Li                                                    [Page 1]
RFC 1518          CIDR Address Allocation Architecture    September 1993

   There are two aspects of interest when discussing IP address
   allocation within the Internet. The first is the set of
   administrative requirements for obtaining and allocating IP
   addresses; the second is the technical aspect of such assignments,
   having largely to do with routing, both within a routing domain
   (intra-domain routing) and between routing domains (inter-domain
   routing). This paper focuses on the technical issues.

   In the current Internet many routing domains (such as corporate and
   campus networks) attach to transit networks (such as regionals) in
   only one or a small number of carefully controlled access points.
   The former act as subscribers, while the latter act as providers.

   The architecture and recommendations provided in this paper are
   intended for immediate deployment. This paper specifically does not
   address long-term research issues, such as complex policy-based
   routing requirements.

   Addressing solutions which require substantial changes or constraints
   on the current topology are not considered.

   The architecture and recommendations in this paper are oriented
   primarily toward the large-scale division of IP address allocation in
   the Internet. Topics covered include:

      - Benefits of encoding some topological information in IP
        addresses to significantly reduce routing protocol overhead;

      - The anticipated need for additional levels of hierarchy in
        Internet addressing to support network growth;

      - The recommended mapping between Internet topological entities
        (i.e., service providers, and service subscribers) and IP
        addressing and routing components;

      - The recommended division of IP address assignment among service
        providers (e.g., backbones, regionals), and service subscribers
        (e.g., sites);

      - Allocation of the IP addresses by the Internet Registry;

      - Choice of the high-order portion of the IP addresses in leaf
        routing domains that are connected to more than one service
        provider (e.g., backbone or a regional network).

   It is noted that there are other aspects of IP address allocation,
   both technical and administrative, that are not covered in this
   paper.  Topics not covered or mentioned only superficially include:

Rekhter & Li                                                    [Page 2]
RFC 1518          CIDR Address Allocation Architecture    September 1993

      - Identification of specific administrative domains in the
        Internet;

      - Policy or mechanisms for making registered information known to
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