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rfc:bcp:bcp4

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.
  1. —————————————————————
 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
  1. —————————————————————

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

Nesser Best Current Practice [Page 10]

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