RFC 1917

Network Working Group                                       P. Nesser II
Request for Comments: 1917                    Nesser & Nesser Consulting
BCP: 4                                                     February 1996
Category: Best Current Practice

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

Status of this Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.


   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 Assigned Numbers 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 reapportionment.

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.

   The Internet is currently faced with two major operational problems
   (amoung others).  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.

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RFC 1917      Appeal to Return Unused IP Networks to IANA  February 1996

   The initial design of IP had all addresses the same, eight bits of
   network number and twenty four bits of host number.  The expectation
   was of a few, large, global networks.  During the first spurts of
   growth, especially with the invention of LAN technologies, it became
   obvious that this assumption was wrong and the separation of the
   address space into three classes (Class A for a few huge networks;
   Class B for more, smaller networks; and Class C for those really
   small LANs, with lots of network numbers) was implemented.  Soon
   subnets were added so sites with many small LANs could appear as a
   single network to others, the first step at limiting routing table
   size.  And finally, CIDR was introduced to the network, to add even
   more flexibility to the addressing, extending the split from three
   classes to potentially thirty different classes.

   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 as

   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 (i.e. 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.

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   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 described a technical experiment designed to test the
   problems with allocating the currently reserved Class A network
   space.  RFC 1879 described the results of this experiment.  This
   effort shows that "supersubnetting" of a Class A network into
   numerous (even millions) of smaller networks is practical.

   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
   resources, 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 every 9 months.  If routes continue to be
   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

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RFC 1917      Appeal to Return Unused IP Networks to IANA  February 1996


   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

   An additional area of route introduction comes from non-aggregating
   router configurations.  Aggregation is not automatic on most routers,
   and providers who may have intact CIDR blocks are, in many cases,
   advertising individual routes instead of an aggregate block without

   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.
   Similarly, the phrase or any references to the "Core Routers" refer
   to the set of routers which carry the full set of route
   advertisements and act as interconnect points for the public networks
   making up the "Global Internet."

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

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.

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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 lower 6.25% is
   allocated for multicast applications (mbone, OSPF, etc.) and the
   upper half is reserved for future applications.

2.5 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

3. 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 IANA.

   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

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RFC 1917      Appeal to Return Unused IP Networks to IANA  February 1996

   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
   address 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

4. 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 network 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.

4.1 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

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RFC 1917      Appeal to Return Unused IP Networks to IANA  February 1996

   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

   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 "Net 39" experiment as outlined in RFC 1797 and summarized in RFC
   1879 provided 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 or
   the appropriate registry.

   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.

   Since the first introduction of CIDR in April of 1994, many providers
   have aggresively pursued the concepts of aggregation.  Some providers
   actively persuaded their customers to renumber, while others pursued
   peering arrangements with other providers, and others did both.
   Providers should continue to actively and routinely pursue both
   methods to streamline routing table growth.  Cooperation between
   providers is absolutely essential to short (and long) term management
   of routing requirements.

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RFC 1917      Appeal to Return Unused IP Networks to IANA  February 1996

   Providers should regularly verify the routes they are advertising to
   their upstream provider(s) to validate their router configurations
   and confirm correct aggregation is occuring.

4.2 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.

4.3 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 returned.


   Please update the contact information on the following net as

   Netname: RESERVED
   Netnumber: $NET_PREFIX

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


4.4 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.

5. 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 losing routes will be forced to make
   one of two choices, both painful to the user community.

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RFC 1917      Appeal to Return Unused IP Networks to IANA  February 1996

   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 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 organizations.

   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.

6. References

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

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

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

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

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

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

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

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RFC 1917      Appeal to Return Unused IP Networks to IANA  February 1996

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

7. Security Considerations

   Security issues are not discussed in this memo.

8. Acknowledgements

   I would like to thank the members of the CIDRD mailing list and
   working groups for their suggestion and comments on this document.
   Specific thanks should go to Michael Patton, Tony Li, Noel Chiappa,
   and Dale Higgs for detailed comments and suggestions.

9. Author's Address

   Philip J. Nesser II
   Nesser & Nesser Consulting
   16015 84th Avenue N.E.
   Bothell, WA 98011-4451

   Phone: (206)488-6268
   Fax: (206)488-6268
   EMail: pjnesser@martigny.ai.mit.edu

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