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+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.
+
+Abstract
+
+   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.
+
+
+
+
+
+Nesser                   Best Current Practice                  [Page 1]
+
+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 255.0.0.0 (in the same way a 16 bit
+   subnet mask will be written as 255.255.0.0).
+
+   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 192.24.0.0 and ending
+   with 192.31.255.0 can be referenced as 192.24/19, or 192.24.0.0 with
+   a mask of 255.248.0.0 (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.
+
+
+
+
+Nesser                   Best Current Practice                  [Page 2]
+
+RFC 1917      Appeal to Return Unused IP Networks to IANA  February 1996
+
+
+   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
+
+
+
+Nesser                   Best Current Practice                  [Page 3]
+
+RFC 1917      Appeal to Return Unused IP Networks to IANA  February 1996
+
+
+   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.
+
+   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
+   realizing.
+
+   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
+   requests.
+
+2.1 Class A Networks (/8 Prefixes)
+
+   The upper half of the Class A address space (64.0.0.0 - 126.0.0.0)
+   (127.0.0.0 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.
+
+
+
+
+Nesser                   Best Current Practice                  [Page 4]
+
+RFC 1917      Appeal to Return Unused IP Networks to IANA  February 1996
+
+
+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
+   century.
+
+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
+   assignments.
+
+
+
+
+Nesser                   Best Current Practice                  [Page 5]
+
+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
+   networks.
+
+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
+
+
+
+Nesser                   Best Current Practice                  [Page 6]
+
+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
+   standing.
+
+   For example, large organization S has been assigned the class A block
+   of addresses 10.0.0.0. and is currently using provider P for their
+   connection to the global Internet.  P is already advertising the
+   route for 10.0.0.0 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
+   schemes.
+
+   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.
+
+
+
+
+
+
+Nesser                   Best Current Practice                  [Page 7]
+
+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
+   follows:
+
+   Netname: RESERVED
+   Netnumber: $NET_PREFIX
+
+   Coordinator:
+     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
+    Registry
+
+   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.
+
+
+
+Nesser                   Best Current Practice                  [Page 8]
+
+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.
+
+
+
+
+Nesser                   Best Current Practice                  [Page 9]
+
+RFC 1917      Appeal to Return Unused IP Networks to IANA  February 1996
+
+
+        8.  IANA, Class A Subnet Experiment, RFC 1797, April
+            1995.
+
+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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+Nesser                   Best Current Practice                 [Page 10]
+