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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
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+
+ 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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