Network Working Group P. Nesser II
Request for Comments: 1917 Nesser & Nesser Consulting
BCP: 4 February 1996
Category: Best Current Practice
An Appeal to the Internet Community to Return
Unused IP Networks (Prefixes) to the IANA
Status of this Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
This document is an appeal to the Internet community to return unused
address space, i.e. any block of consecutive IP prefixes, to the
Internet Assigned Numbers Authority (IANA) or any of the delegated
registries, for reapportionment. Similarly an appeal is issued to
providers to return unused prefixes which fall outside their
customary address blocks to the IANA for reapportionment.
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.
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 18.104.22.168 and ending
with 22.214.171.124 can be referenced as 192.24/19, or 126.96.36.199 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.
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
The second source for new route introduction is sites who had
previously operated a private IP network, which had been registered
and assigned a network number (or numerous networks), but have only
recently connected to the global Internet. This RFC is a policy
based attempt to help preserve the operation of the current Internet
by addressing the issues of previously registered but unannounced IP
An additional area of route introduction comes from non-aggregating
router configurations. Aggregation is not automatic on most routers,
and providers who may have intact CIDR blocks are, in many cases,
advertising individual routes instead of an aggregate block without
In the context of this document, the phrase "Global Internet" refers
to the mesh of interconnected public networks (Autonomous Systems)
which has its origins in the U.S. National Science Foundation (NSF)
backbone, other national networks, and commercial enterprises.
Similarly, the phrase or any references to the "Core Routers" refer
to the set of routers which carry the full set of route
advertisements and act as interconnect points for the public networks
making up the "Global Internet."
The IANA has historically managed the assignment of addresses to
Internet sites. During the earliest days of the IANA, given a vast
address space, the requirements for assignments of network address
space were much less stringent than those required today.
Organizations were essentially assigned networks based on their
2.1 Class A Networks (/8 Prefixes)
The upper half of the Class A address space (188.8.131.52 - 184.108.40.206)
(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.
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.
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
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
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
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
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
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
their address space (assuming they are subnetting using contiguous
bits) may be willing to allow usage of the upper portion of their
assigned address space by their providers other customers.
This scheme requires certain elements be installed or already in
place to get the routing correct, but has the potential to gain the
use of a large number of small networks without growth of the global
routing tables. This would require additional measures of
cooperation between providers and their customers but could prove to
have both economic advantages, as well as good Internet citizen
For example, large organization S has been assigned the class A block
of addresses 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
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.
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 & email@example.com,
changing the $NET_PREFIX to the network being returned.
Please update the contact information on the following net as
Reynolds, Joyce K. (JKR1) JKRey@ISI.EDU
Postel, Jon (JBP) POSTEL@ISI.EDU
4.4 How to Return a Block of Address Space to another Address
Each registry will have its own forms and addresses. Please contact
the appropriate registry directly.
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.
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.
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.
8. IANA, Class A Subnet Experiment, RFC 1797, April
7. Security Considerations
Security issues are not discussed in this memo.
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