Username / Password : Request For Comments

RFC Number : 1900

Title : Renumbering Needs Work.

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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