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CIDRD Working Group                      Philip J. Nesser II
Internet Draft                        Olin Aerospace Company
Expires in six months                            August 1995

        An Appeal to the Internet Community to Return
           Unused IP Networks(Prefixes) to the IANA

Status of this Memo

     This document is an Internet Draft.  Internet Drafts
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This document is an appeal to the Internet community to return
unused address space, i.e. any block of consecutive IP prefixes,
to the Internet Address Naming Authority (IANA) or any of the
delegated registries, for reapportionment.  Similarly an appeal
is issued to providers to return unused prefixes which fall
outside their customary address blocks to the IANA for

1. Background

The Internet of today is a dramatically different network than
the original designers ever envisioned.  It is the largest
public data network in the world, and continues to grow at an
exponential rate which doubles all major operational parameters
every nine months.  A common metaphor in engineering is that
every time a problem increases in size by an order of magnitude,
it becomes a new problem.  This adage has been true over the
lifetime of the Internet.  A simple example is the notion of
classful addresses.  The A, B, C, classes of networks which are
inherent in the initial design of IP are clearly inappropriate
for the Internet of today.

The Internet is currently faced with two major operational
problems.  The first is the eventual exhaustion of the IPv4
address space and the second is the ability to route packets
between the large number of individual networks that make up the
Internet.  The first problem is simply one of supply.  There are
only 2^32 IPv4 addresses available.  The lifetime of that space
is proportional to the efficiency of its allocation and
utilization.  The second problem is mainly a capacity problem.
If the number of routes exceeds the current capacity of the core
Internet routers, some routes will be dropped and sections of
the Internet will no longer be able to communicate with each
other.  The two problems are coupled and the dominant one has,
and will, change over time.

Subnets were introduced to provide a mechanism for sites to
divide a single network number (Class A, B, or C) into pieces,
allowing a higher utilization of address space, and thus
promoting conservation of the IPv4 address space.  Because of
the built-in notion of classful addresses, subnetting
automatically induced a reduction in the routing requirements on
the Internet.  Instead of using two (or more) class C networks,
a site could subnet a single class B into two (or more) subnets.
Both the allocation and the advertisement of a route to the
second and succeeding class C's are saved.

Since 1993, the concept of classless (the "C" in CIDR) addresses
have been introduced to the Internet community.  Addresses are
increasingly thought of as bitwise contiguous blocks of the
entire address space, rather than a class A,B,C network.  For
example, the address block formerly known as a Class A network,
would be referred to as a network with a /8 prefix, meaning the
first 8 bits of the address define the network portion of the
address.  Sometimes the /8 will be expressed as a mask of (in the same way a 16 bit subnet mask will be written

This scheme allows "supernetting" of addresses together into
blocks which can be advertised as a single routing entry.  The
practical purpose of this effort is to allow service providers
and address registries to delegate realistic address spaces to
organizations and be unfettered by the traditional network
classes, which were inappropriately sized for most
organizations.  For example the block of 2048 class C network
numbers beginning with and ending with
can be referenced as 192.24/19, or with a mask of (ie similar to a 19 bit subnet mask written in
dotted decimal notation).  The concept of "supernetting" allows
the remaining Internet address space to be allocated in smaller
blocks, thus allowing more networks and better efficiency.  For
a more detailed discussion refer to RFC 1518.

Like subnetting, CIDR also helps address the reduction of
routing requirements, but it is not as automatic as the case of
subnets.  CIDR blocks are allocated in a way which promotes
hierarchical routing.  A provider is typically given a large
block of addresses to redistribute to their customers.  For
example, if the provider P has been given the CIDR block
192.168/16, a block of 255 contiguous class C networks, they can
provide one class C network to each of 255 customers (who may in
turn subnet those class C networks into smaller pieces) yet
still only advertise the single route 192.168/16.  Thus CIDR
only helps reduce the routing problem if blocks are assigned and
maintained in a hierarchical manner.

RFC 1797 is a technical experiment designed to test the
problems with allocating the currently reserved Class A network
space.  This effort would allow "supersubnetting" of a Class A
network into numerous (even millions) of smaller networks.

The dominating portion of the problem facing the Internet today
is routing requirements.  The following statements constitute a
first order approximation based on current growth, a simple
model of router resouces, etc.  Current routing technology can
handle approximately twice the number of routes which are
currently advertised on "core" Internet routers.  Router
capacity is doubling every 18 months, while routing tables are
doubling ever 9 months.  If routes are introduced at the current
rate, the Internet will cease to function as a reliable
infrastructure in approximately 2 to 3 years.

The good news is that CIDR is working.  Address blocks are being
allocated and assigned in a hierarchical manner, and the
CIDR'ization of large portions of the address space which were
assigned according to the guidelines of RFC 1466 resulted in a
significant drop of advertised routes.  However, recent growth
trends show that the number of routes is once again growing at
an exponential rate, and that the reduction with the
introduction of CIDR was simply a sawtooth in the rate.

The growth in the number of routes can logically come from only
two places, the extra routes generated with the breakup of CIDR
blocks, and previously allocated and unannounced networks being
connected.  (Registries are still allocating a few addresses not
within CIDR blocks, so a small third source does exist.)  With
increasing popularity there is increasing competition between
providers.  If a site changes provider and retains the use of
their CIDR block addresses, holes appear in the blocks and
specific routes are added to the routing structure to
accommodate these cases.  Thus over time, CIDR will improve
address utilization efficiency yet not help the routing
requirements unless providers can keep their CIDR blocks intact.

The second source for new route introduction is sites who had
previously operated a private IP network, which had been
registered and assigned a network number (or numerous networks),
but have only recently connected to the global Internet.  This
RFC is a policy based attempt to help preserve the operation of
the current Internet by addressing the issues of previously
registered but unannounced IP networks.

In the context of this document, the phrase "Global Internet"
refers to the mesh of interconnected public networks (Autonomous
Systems) which has its origins in the U.S. National Science
Foundation (NSF) backbone, other national networks, and
commercial enterprises.

2. History

The IANA has historically managed the assignment of addresses to
Internet sites.  During the earliest days of the IANA, given a
vast address space, the requirements for assignments of network
address space were much less stringent than those required
today.  Organizations were essentially assigned networks based
on their requests.

2.1 Class A Networks (/8 Prefixes)

The upper half of the Class A address space ( - ( has traditionally been used by the Unix
operating system as the "loopback" network, and is thus
unavailable) has been reserved by the IANA for growth within the
IPv4 address space.  Of the lower half of the address space, 22
were assigned pre-1982, 6 were assigned between 1982 and 1987,
26 were assigned between 1988 and 1992, and 2 were assigned
between 1993 and 1995.  In May of 1995 four Class A networks
previously assigned have been returned to the IANA.  All
remaining Class A addresses have also been reserved for growth
within the IPv4 address space. The Class A address space is 50%
of the total IPv4 address space.

2.2 Class B Networks (/16 prefixes)

>From 1989 until 1993 approximately 80% of the currently assigned
Class B IP networks were assigned or allocated.  Allocations
dropped dramatically in 1994 and 1995 due to the adoption of
policies outlined in RFC 1466.  61.65% of the Class B address
space is currently allocated.  The class B address space is 25%
of the total IPv4 address space.

2.3 Class C Networks (/24 Prefixes)

With the introduction of CIDR and RFC 1466 the allocation of
Class C address space has skyrocketed since 1993.  27.82% of the
Class C address space is currently allocated.  The class C
address space is 12.5% of the total IPv4 address space.

2.4 Class "D" and Beyond

Of the remaing 12.5% of the address space, the upper 6.25% is
allocated for multicast applications (mbone, OSPF, etc.) and the
lower half is reserved for future applications.

2.4 Totals

The weighted total shows that 40.99% of the total IPv4 address
space is allocated and the remainder is reserved for future
growth. It should be noted that careful extrapolations of the
current trends suggest that the address space will be exhausted
early in the next century.

4. Problem

Before the introduction of RFC 1466 and of CIDR, some 50,000
networks were assigned by the IANA, yet only a small percentage
(30-40%) of the sites actually had connections to the global
Internet and advertised those networks.  As the popularity of
the Internet is growing, a growing number of those sites are
being connected, and increasing the size of the routing tables.

Current Internet sites have received their address assignments
in various ways and steps.  Some sites, through a little (or in
some cases no) work, could donate unused IP nets back to the

Some organizations have made small requests at first and
received a Class C assignment (or multiple Class C assignments),
and after unexpected growth made subsequent requests and
received Class B assignments.

Several Internet service providers were given blocks of the
Class B address space to distribute to customers.  This space
was often provided to clients based upon a level of service
purchased rather than actual need.

Many organizations have either merged or are associated with
parent organizations which produce situations with large
inefficiencies in address assignment.

Many organizations have requested addresses based on their need
to run TCP/IP on internal machines which have no interest in
connecting to the global Internet.  Most vendors manuals have
instructed (and provided copies of the application forms), sites
to request IP addresses assignments.

Other organizations have large internal IP networks, and are
connected to the Internet through application layer gateways or
network address translators, and will never announce their
internal networks.

5. Appeal

To the members of the Internet community who have IP network
assignments which may be currently unused, the Internet
community would like to encourage you to return those addresses
to the IANA or your provider for reapportionment.

Specifically those sites who have networks which are unused are
encouraged to return those addresses. Similarly to those sites
who are using a small percentage of their address space and who
could relatively easily remove networks assignments from active
use, the Internet community encourages such efforts.

To those sites who have networks which will never need to
connect to the global Internet, or for security reasons will
always be isolated, consider returning the address assignments
to the IANA or your provider and utilizing prefixes recommended
in RFC 1597.

In those cases where renumbering is required, sites are
encouraged to put into place a plan to renumber machines,
as is reasonably convenient, and work towards minimizing the
number of routes advertised to their providers.

5.1 Suggestions to Providers

Many providers are currently advertising Non-CIDR routes which
encompass a large block of addresses, ie any Class A (0/1) or
Class B (128/2) space.  Some customers who are only using a
percentage of their address space (assuming they are subnetting
using contiguous bits) may be willing to allow usage of the
upper portion of their assigned address space by their providers
other customers.

This scheme requires certain elements be installed or already in
place to get the routing correct, but has the potential to gain
the use of a large number of small networks without growth of
the global routing tables.  This would require additional
measures of cooperation between providers and their customers
but could prove to have both economic advantages, as well as
good Internet citizen standing.

For example, large organization S has been assigned the class A
block of addresses and is currently using provider P
for their connection to the global Internet.  P is already
advertising the route for to the global Internet.  S
has been allocating its internal networks using a right to left
bit incrementing model.  P and S could agree that S will allow
some /18 (for example) prefixes to be made available for P's
other customers.  This would impose no hardships whatsoever on
S, presuming his router can speak BGP, and allow P to attach a
huge number of small customers without the need to advertise
more routes or request additional address blocks from the IANA
or their upstream provider.

The current Net 39 experiment as outlined in RFC 1797 should
provide practical data on the implementation of the suggested

Additionally, providers are encouraged to release all unused
networks which fall outside of their normal address blocks back
to the IANA.

New customers, particularly those who may have recently changed
providers, and who have small networks which are not part of
CIDR'ized blocks, should be encouraged to renumber and release
their previous addresses back to the provider or the IANA.

5.2 Suggestions to the IANA and Address Registries

In cases where addresses are returned to the IANA, or any other
address registry, which fits into another registry or providers
block, the addresses should be turned over to the appropriate
authority.  This will help maximize the availability of
addresses and minimize routing table loads.

5.3 How to Return a Block of Address Space to the IANA

Send the following form to Hostmaster@internic.net &
iana@isi.edu, changing the $NET_PREFIX to the network being


Please update the contact information on the following net as

Netnumber: $NET_PREFIX

  Reynolds, Joyce K.  (JKR1)  JKRey@ISI.EDU
  (310) 822-1511
Alternate Contact:
  Postel, Jon  (JBP)  POSTEL@ISI.EDU
  (310) 822-1511


5.4 How to Return a Block of Address Space to another Address

Each registry will have its own forms and addresses.  Please
contact the appropriate registry directly.

6. Conclusion

Rationalizing the global addressing hierarchy is a goal which
should be supported by any organization which is currently
connected or plans to connect to the Internet.  If (and possibly
when) the situation ever reaches a critical point, the core
service providers whose routers are failing and loosing routes
will be forced to make one of two choices, both painful to the
user community.

They could begin blocking routes to their customers who are
advertising too many disjoint routes, where "too many" will be
set at the level necessary to keep their routers functioning
properly.  This is a domino effect since the next level of
providers will be forced to make the same effort, until
individual organizations are forced to only advertise routes to
portions of their networks.

The second option the core providers have will is to charge for
advertised routes.  The price level will be set at a point which
reduces the number of routes to a level which will keep their
routers functioning properly.  Once again a domino effect will
take place until the price increases will effect individual

Some planning and efforts by organizations and providers now
while there is a some time available can help delay or prevent
either or the two scenarios from occurring.

This system has already produced very favorable results when
applied on a small scale.  As of this writing 4 Class A networks
have been returned to the IANA.  This may not seem significant
but those 4 networks represent over 1.5% of the total IPv4
address capacity.

7. References

        1.  E. Gerich, Guidelines for Management of the IP
            Address Space, RFC 1466, May 1993.

        2.  C. Topolcic, Status of CIDR Deployment in the
            Internet, RFC 1467, August 1993.

        3.  Y. Rekhter, T. Li, An Architecture for IP Address
            Allocation with CIDR, RFC 1518, September 1993.

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

        5.  Y. Rekhter, R. Moskowitz, D. Karrenberg, G. de
            Groot, Address Allocation for Private Internets, RFC
            1597, March 1994.

        6.  E. Lear, E. Fair, D. Crocker, T. Kessler, Network 10
            Considered Harmful (Some Practices Shouldn't be
            Codified), RFC 1627, July 1994.

        7.  C. Huitema, The H Ratio for Address Assignment
            Efficiency, RFC 1715, November 1994.

        8.  IANA, Class A Subnet Experiment, RFC 1797, April

8. Security Consideration

Security issues are not discussed in this memo.

9. Authors Address

Philip J. Nesser II
Olin Aerospace Company
11441 Willows Road N.E.
Redmond, WA 98052
Phone: (206) 885-5000, extension 5477
Fax: (206) 882-5750
EMail: pjnesser@rocket.com

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