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   addressing information - such allocation is called "aggregator
   based". To benefit from the "aggregator based" address allocation,
   CIDR introduces an inter-domain routing protocol (BGP-4) [RFC 1771,
   RFC 1772] that provides capabilities for routing information
   aggregation at the level of individual sites and providers.

   CIDR improves address space utilization by eliminating the notion of
   network classes,  and replacing it with the notion of contiguous
   variable size (power of 2) address blocks. This allows a better match
   between the amount of address space requested and the amount of
   address space allocated [RFC 1466]. It also facilitates "aggregator
   based" address allocation. Eliminating the notion of network classes
   requires new capabilities in the routing protocols (both intra and
   inter-domain), and IP forwarding. Specifically, the CIDR-capable



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   protocols are required to handle reachability (addressing)
   information expressed in terms of variable length address prefixes,
   and forwarding  is required to implement the "longest match"
   algorithm.  CIDR implications on routing protocols are described in
   [RFC 1817].

   The scaling capabilities of CIDR are based on the assumption that
   address allocation reflects network topology as much as possible,
   especially at the level of sites, and their interconnection with
   providers, to enable sites and providers to act as aggregators. If a
   site changes its provider, then to avoid injecting additional
   overhead in the Internet routing system, the site may need to
   renumber. While CIDR does not require every site that changes its
   providers to renumber, it is important to stress that if none of the
   sites that change their providers will renumber, the Internet routing
   system might collapse due to the excessive amount of routing
   information it would need to handle.

   Maintaining "aggregator based" address allocation (to promote
   scalable routing), and the need to support the ability of sites to
   change their providers (to promote competition) demands practical
   solutions for renumbering sites.  The need to contain the  overhead
   in a rapidly growing Internet routing system is likely to make
   renumbering  more and more common [RFC 1900].

   The need to scale the Internet routing system, and the use of CIDR as
   the primary mechanism for scaling, results in the evolution of
   address allocation and management policies for the Internet. This
   evolution results in adding the "address lending" policy as an
   alternative to the "address ownership" policy [RFC 2008].

   IP addressing and routing have been in constant evolution since IP
   was first specified [RFC 791]. Some of the addressing and routing
   principles have been deprecated, some of the principles have been
   preserved, while new principles have been introduced. Current
   Internet routing and addresses (based on CIDR) is an evolutionary
   step that extends the use of hierarchy to maintain a routable global
   Internet.

Security Considerations

   The impact of the IP addressing model on security is discussed in
   sections 4.1 and 5 of this document.








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Acknowledgements

   This document was developed in the IAB. Constructive comments were
   received from Ran Atkinson, Jim Bound, Matt Crawford, Tony Li,
   Michael A. Patton, Jeff Schiller. Earlier private communications from
   Noel Chiappa helped to clarify the concepts of locators and
   identifiers.

References

   [RFC 791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
   1981.

   [RFC 790] Postel, J., "Assigned Numbers", September 1981.

   [RFC 959] Postel, J., and J. Reynolds, "File Transfer Protocol", STD
   9, RFC 959, October 1985.

   [RFC 1035] Mockapetris, P., "Domain Names - Implementation and
   Specification", STD 13, RFC 1035, November 1987.

   [RFC 1112] Deering, S., "Host Extensions for IP Multicasting", STD 5,
   RFC 1112, September 1989.

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

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

   [RFC 1498] Saltzer, J., "On the Naming and Binding of Network
   Destinations", RFC 1498, August 1993 (originally published 1982).

   [RFC 1518] Rekhter, Y., and T. Li, "An Architecture for IP Address
   Allocation with CIDR", RFC 1518, September 1993.

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

   [RFC 1541] Droms, R., "Dynamic Host Configuration Protocol", RFC
   1541, October 1993.

   [RFC 1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
   RFC 1661, July 1994.

   [RFC 1771] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4
   (BGP-4)", RFC 1771, March 1995.



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   [RFC 1772] Rekhter, Y., and P. Gross, "Application of the Border
   Gateway Protocol in the Internet", RFC 1772, March 1995.

   [RFC 1817] Rekhter, Y., "CIDR and Classful Routing", RFC 1817,
   September 1995.

   [RFC 1825] Atkinson, R., "Security Architecture for the Internet
   Protocol", RFC 1825, September 1995.

   [RFC 1900] Carpenter, B., and Y. Rekhter, "Renumbering Needs Work",
   RFC 1900, February 1996.

   [RFC 1918] Rekhter, Y.,  Moskowitz, B., Karrenberg, D., de Groot, G.
   J., and E. Lear, "Address Allocation for Private Internets", RFC
   1918, February 1996.

   [RFC 1933] Gilligan, R., and E. Nordmark, "Transition Mechanisms for
   IPv6 Hosts and Routers", RFC 1933, April 1996.

   [RFC 2008] Rekhter, Y., and T. Li, "Implications of  Various Address
   Allocation Policies for Internet Routing", RFC 2008, October 1996.

   [kre] Elz, R., et. al., "Selection and Operation of Secondary DNS
   Servers", Work in Progress.

   [RFC 2065] Eastlake, E., and C. Kaufman, "Domain Name System Security
   Extensions", RFC 2065, January 1997.

   [dns2] Vixie, P., et. al., "Dynamic Updates in the Domain Name System
   (DNS UPDATE)", Work in Progress.





















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Authors' Addresses

   Brian E. Carpenter
   Computing and Networks Division
   CERN
   European Laboratory for Particle Physics
   1211 Geneva 23, Switzerland

   EMail: [e-mail]

   Jon Crowcroft
   Dept. of Computer Science
   University College London
   London WC1E 6BT, UK

   EMail: [e-mail]

   Yakov Rekhter
   Cisco systems
   170 West Tasman Drive
   San Jose, CA, USA

   Phone: [phone]
   Fax: [phone]
   EMail: [e-mail]


























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