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Network Working Group                                       P. Nesser IIRequest for Comments: 1917                    Nesser & Nesser ConsultingBCP: 4                                                     February 1996Category: Best Current PracticeAn Appeal to the Internet Community to ReturnUnused IP Networks (Prefixes) to the IANAStatus 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 toRFC 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 ofRFC 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 blocksNesser                   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 19962.2 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 inRFC 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 andRFC 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 ofRFC 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 inRFC 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 ofNesser                   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 inRFC 1797 and summarized inRFC1879 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.eduNesser                   Best Current Practice                 [Page 10]