Network Working Group                                       B. Carpenter
Request for Comments: 1900                                    Y. Rekhter
Category: Informational                                              IAB
                                                           February 1996

                         Renumbering Needs Work

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.


   Renumbering, i.e., changes in the IP addressing information of
   various network components, is likely to become more and more
   widespread and common. The Internet Architecture Board (IAB) would
   like to stress the need to develop and deploy solutions that would
   facilitate such changes.

Table of Contents

   1. Motivation................................................... 1
   2. DNS versus IP Addresses...................................... 2
   3. Recommendations.............................................. 3
   4. Security Considerations...................................... 4
   Acknowledgements................................................ 4
   Authors' Addresses.............................................. 4

1. Motivation

   Hosts in an IP network are identified by IP addresses, and the IP
   address prefixes of subnets are advertised by routing protocols.  A
   change in such IP addressing information associated with a host or
   subnet is known as "renumbering".

   Renumbering may occur for a variety of reasons.  For example, moving
   an IP host from one subnet to another requires changing the host's IP
   address.  Physically splitting a subnet due to traffic overload may
   also require renumbering.  A third example where renumbering may
   happen is when an organization changes its addressing plan.  Such
   changes imply changing not only hosts' addresses, but subnet numbers
   as well.  These are just three examples that illustrate possible
   scenarios where renumbering could occur.

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RFC 1900                 Renumbering Needs Work            February 1996

   Increasingly, renumbering will be needed for organizations that
   require Internet-wide IP connectivity, but do not themselves provide
   a sufficient degree of address information aggregation.  Unless and
   until viable alternatives are developed, extended deployment of
   Classless Inter-Domain Routing (CIDR) is vital to keep the Internet
   routing system alive and to maintain continuous uninterrupted growth
   of the Internet.  With current IP technology, this requires such
   organizations to use addresses belonging to a single large block of
   address space, allocated to their current service provider which acts
   as an aggregator for these addresses.  To contain the growth of
   routing information, whenever such an organization changes to a new
   service provider, the organization's addresses will have to change.
   Occasionally, service providers themselves may have to change to a
   new and larger block of address space. In either of these cases, to
   contain the growth of routing information, the organizations
   concerned would need to renumber their subnet(s) and host(s). If the
   organization does not renumber, then some of the potential
   consequences may include (a) limited (less than Internet-wide) IP
   connectivity, or (b) extra cost to offset the overhead associated
   with the organization's routing information that Internet Service
   Providers have to maintain, or both.

   Currently, renumbering is usually a costly, tedious and error-prone
   process.  It normally requires the services of experts in the area
   and considerable advance planning.  Tools to facilitate renumbering
   are few, not widely available, and not widely deployed. While a
   variety of ad hoc approaches to renumbering have been developed and
   used, the overall situation is far from satisfactory.  There is
   little or no documentation that describes renumbering procedures.
   While renumbering occurs in various parts of the Internet, there is
   little or no documented experience sharing.

2. DNS versus IP Addresses

   Within the Internet architecture an individual host can be identified
   by the IP address(es) assigned to the network interface(s) on that
   host.  The Domain Name System (DNS) provides a convenient way to
   associate legible names with IP addresses.  The DNS name space is
   independent of the IP address space.  DNS names are usually related
   to the ownership and function of the hosts, not to the mechanisms of
   addressing and routing.  A change in DNS name may be a sign of a real
   change in function or ownership, whereas a change in IP address is a
   purely technical event.

   Expressing information in terms of Domain Names allows one to defer
   binding between a particular network entity and its IP address until
   run time. Domain Names for enterprises, and Fully Qualified Domain
   Names (FQDNs, see RFC 1594) for servers and many user systems, are

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RFC 1900                 Renumbering Needs Work            February 1996

   expected to be fairly long-lived, and more stable than IP addresses.
   Deferring the binding avoids the risk of changed mapping between IP
   addresses and specific network entities (due to changing addressing
   information).  Moreover, reliance on FQDNs (rather than IP addresses)
   also localizes to the DNS the changes needed to deal with changing
   addressing information due to renumbering.

   In some cases, both the addresses and FQDNs of desk top or portable
   systems are allocated dynamically. It is only a highly responsive
   dynamic DNS update mechanism that can cope with this.

3. Recommendations

   To make renumbering more feasible, the IAB strongly recommends that
   all designs and implementations should minimise the cases in which IP
   addresses are stored in non-volatile storage maintained by humans,
   such as configuration files.  Configuration information used by
   TCP/IP protocols should be expressed, whenever possible, in terms of
   Fully Qualified Domain Names, rather than IP addresses. Hardcoding IP
   addresses into applications should be deprecated.  Files containing
   lists of name to address mappings, other than that used as part of
   DNS configuration, should be deprecated, and avoided wherever

   There are times when legacy applications which require configuration
   files with IP addresses rather than Domain Names cannot be upgraded
   to meet these recommendations. In those cases, it is recommended that
   the configuration files be generated automatically from another file
   which uses Domain Names, with the substitution of addresses being
   done by lookup in the DNS.

   Use of licensing technology that is based upon the IP address of a
   host system makes renumbering quite difficult. Therefore, the use of
   such technology should be strongly discouraged.

   The development and deployment of a toolkit to facilitate and
   automate host renumbering is essential.  The Dynamic Host
   Configuration Protocol (DHCP) is clearly an essential part of such a
   toolkit.  The IAB strongly encourages implementation and wide-scale
   deployment of DHCP.  Dynamic router discovery (RFC 1256) and service
   location (work in progress in the IETF) also belong in this toolkit.
   Support for dynamic update capabilities to the Domain Name System
   (DNS) that could be done with sufficient authentication would further
   facilitate host renumbering.  The IAB strongly encourages progression
   of work in this area towards standardization within the IETF, with
   the goal of integrating DHCP and dynamic update capabilities to
   provide truly autoconfigurable TCP/IP hosts.

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RFC 1900                 Renumbering Needs Work            February 1996

   The IAB strongly encourages sharing of experience with renumbering
   and documenting this sharing within the Internet community.  The IAB
   suggests that the IETF (and specifically its Operational Requirements
   Area) may be the most appropriate place to develop such
   documentation.  The IAB welcomes the creation of the PIER (Procedures
   for Internet and Enterprise Renumbering) working group.

4. Security Considerations

   Renumbering is believed to be compatible with the Internet security
   architecture, as long as addresses do not change during the lifetime
   of a security association.


   This document is a collective product of the Internet Architecture

   Useful comments were received from several people, especially Michael
   Patton, Steve Bellovin, Jeff Schiller, and Bill Simpson.

Authors' Addresses

   Brian E. Carpenter
   Group Leader, Communications Systems
   Computing and Networks Division
   European Laboratory for Particle Physics
   1211 Geneva 23, Switzerland

   Phone:  +41 22 767-4967
   Fax:    +41 22 767-7155
   Telex:  419000 cer ch

   Yakov Rekhter
   cisco Systems
   170 West Tasman Drive
   San Jose, CA 95134

   Phone: (914) 528-0090

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