|Title||Guidelines for Management of IP Address Space
Network Working Group E. Gerich
Request for Comments: 1366 Merit
Guidelines for Management of IP Address Space
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard. Distribution of this memo is
This document has been reviewed by the Federal Engineering Task Force
(FEPG) on behalf of the Federal Networking Council (FNC), the co-
chairs of the International Engineering Planning Group (IEPG), and
the Reseaux IP Europeens (RIPE). There was general consensus by
those groups to support the recommendations proposed in this document
for management of the IP address space.
With the growth of the Internet and its increasing globalization,
much thought has been given to the evolution of the network number
allocation and assignment process. RFC 1174, "Identifier Assignment
and Connected Status", dated August 1990 recommends that the Internet
Registry (IR) continue as the principal registry for network numbers;
however, the IR may allocate blocks of network numbers and the
assignment of those numbers to qualified organizations. The IR will
serve as the default registry in cases where no delegated
registration authority has been identified.
The distribution of the registration function is desirable, and in
keeping with that goal, it is necessary to develop a plan which
manages the distribution of the network number space. The demand for
network numbers has grown significantly within the last two years and
as a result the allocation of network numbers must be approached in a
more systematic fashion.
This document proposes a plan which will forward the implementation
of RFC 1174 and which defines the allocation and assignment of the
network number space. There are three major topics to be addressed:
1) Qualifications for Distributed Regional Registries
2) Allocation of the Network Number Space by the Internet Registry
3) Assignment of the Network Numbers
2.0 Qualifications for Distributed Regional Registries
The major reason to distribute the registration function is that the
Internet serves a more diverse global population than it did at its
inception. This means that registries which are located in distinct
geographic areas may be better able to serve the local community in
terms of language and local customs. While there appears to be wide
support for the concept of distribution of the registration function,
it is important to define how the candidate delegated registries will
be chosen and from which geographic areas.
Based on the growth and the maturity of the Internet in Europe,
Central/South America and the Pacific Rim areas, it is desirable to
consider delegating the registration function to an organization in
each of those geographic areas. Until an organization is identified
in those regions, the IR will continue to serve as the default
registry. The IR remains the root registry and continues to provide
the registration function to all those regions not covered by
distributed regional registries. And as other regions of the world
become more and more active in the Internet, the IANA and the IR may
choose to look for candidate registries to serve the populations in
those geographic regions.
It is important that the regional registry is unbiased and and widely
recognized by network providers and subscribers within the geographic
region. It is also important that there is just a single regional
registry per geographical region at this level to provide for
efficient and fair sub-allocation of the address space. To be
selected as a distributed regional registry an organization should
meet the following criteria:
a) networking authorities within the geographic area
legitimize the organization
b) the organization is well-established and has
legitimacy outside of the registry function
c) the organization will commit appropriate resources to
provide stable, timely, and reliable service
to the geographic region
d) the commitment to allocate IP numbers according to
the guidelines established by the IANA and the IR
e) the commitment to coordinate with the IR to establish
qualifications and strategies for sub-allocations of
the regional allocation.
The distributed regional registry is empowered by the IANA and the IR
to provide the network number registration function to a geographic
area. It is possible for network subscribers to contact the IR
directly. Depending on the circumstances the network subscriber may
be referred to the regional registry, but the IR will be prepared to
service any network subscriber if necessary.
3.0 Allocation of the Network Number Space by the Internet Registry
The Class A portion of the number space represents 50% of the total
IP numbers; Class B is 25% of the total; Class C is approximately 12%
of the total. Table 1 shows the current allocation of the IP network
Total Allocated Allocated (%)
Class A 126 49 38%
Class B 16383 7354 45%
Class C 2097151 44014 2%
Table 1: Network Number Statistics (June 1992) 
Class A and B network numbers are a limited resource and therefore
the entire number space will be retained by the IR. No allocations
from the Class A and B network numbers will be made to distributed
regional registries at this time.
The Class C network number space will be divided into allocatable
blocks which will be reserved by the IANA and IR for allocation to
distributed regional registries. In the absence of designated
regional registries in geographic areas, the IR will assign addresses
to networks within those geographic areas according to the Class C
A preliminary inspection of the Class C IP network numbers shows that
the number space with prefixes 192 and 193 are assigned. The
remaining space from prefix 194 through 223 is mostly unassigned.
The IANA and the IR will reserve the upper half of this space which
corresponds to the IP address range of 220.127.116.11 through
18.104.22.168. Network numbers from this portion of the Class C
space will remain unallocated and unassigned until further notice.
The remaining Class C network number space will be allocated in a
fashion which is compatible with potential address aggregation
techniques. It is intended to divide this address range into eight
equally sized address blocks.
192.0.0.0 - 22.214.171.124
126.96.36.199 - 188.8.131.52
184.108.40.206 - 220.127.116.11
18.104.22.168 - 22.214.171.124
126.96.36.199 - 188.8.131.52
184.108.40.206 - 220.127.116.11
18.104.22.168 - 22.214.171.124
126.96.36.199 - 188.8.131.52
Each block represents 131,072 addresses or approximately 6% of the
total Class C address space.
It is proposed that a broad geographic allocation be used for these
blocks. At present there are four major areas of address allocation:
Europe, North America, Pacific Rim, and South & Central America.
In particular, the top level block allocation be designated as
Multi-regional 192.0.0.0 - 184.108.40.206
Europe 220.127.116.11 - 18.104.22.168
Others 22.214.171.124 - 126.96.36.199
North America 188.8.131.52 - 184.108.40.206
America 220.127.116.11 - 18.104.22.168
Pacific Rim 22.214.171.124 - 126.96.36.199
Others 188.8.131.52 - 184.108.40.206
Others 220.127.116.11 - 18.104.22.168
It is proposed that the IR, and any designated regional registries,
allocate addresses in conformance with this overall scheme. Where
there are qualifying regional registries established, primary
responsibility for allocation from within that block will be
delegated to that registry.
The ranges designated as "Others" permit flexibility in network
number assignments which are outside of the geographical regions
already allocated. The range listed as multi-regional represents
network numbers which have been assigned prior to the implementation
of this plan. It is proposed that the IANA and the IR will adopt
these divisions of the Class C network number space and will begin
assigning network numbers accordingly.
4.0 Assignment of the Network Number Space
The exhaustion of the IP address space is a topic of concern for the
entire Internet community. This plan for the assignment of Class A,
B, or C IP numbers to network subscribers has two major goals:
1) to reserve a portion of the IP number space so that it may be
available to transition to a new numbering plan
2) to assign the Class C network number space in a fashion which
is compatible with proposed address aggregation techniques
4.1 Class A
The Class A number space can support the largest number of unique
host identifier addresses and is also the class of network numbers
most sparsely populated. There are only approximately 77 Class A
network numbers which are unassigned, and these 77 network numbers
represent about 30% of the total network number space.
The IANA will retain sole responsibility for the assignment of Class
A network numbers. The upper half of the Class A number space will be
reserved indefinitely (IP network addresses 22.214.171.124 through
127.0.0.0). While it is expected that no new assignments of Class A
numbers will take place in the near future, any organization
petitioning the IANA for a Class A network number will be expected to
provide a detailed technical justification documenting network size
and structure. Class A assignments are at the IANA's discretion.
4.2 Class B
Previously organizations were recommended to use a subnetted Class B
network number rather than multiple Class C network numbers. Due to
the scarcity of Class B network numbers and the under utilization of
the Class B number space by most organizations, the recommendation is
now to use multiple Class Cs where practical.
The IANA and the IR will maintain sole responsibility for the Class B
number space. Where there are designated regional registries, those
registries will act in an auxiliary capacity in evaluating requests
for Class B numbers. Organizations applying for a Class B network
number should fulfill the following criteria:
1) the organization presents a subnetting plan which
documents more than 32 subnets within its organizational
2) the organization has more than 4096 hosts.
These criteria assume that an organization which meets this profile
will continue to grow and that assigning a Class B network number to
them will permit network growth and reasonable utilization of the
assigned number space. There may be circumstances where it will be
impossible to utilize a block of Class C network numbers in place of
a Class B. These situations will be considered on a case-by-case
4.3 Class C
Section 3 of this document recommends a division of the Class C
number space. That division is primarily an administrative division
which lays the groundwork for distributed network number registries.
This section deals with how network numbers are assigned from within
those blocks. Sub-allocations of the block to sub-registries is
beyond the scope of this paper.
By default, if an organization requires more than a single Class C,
it will be assigned a bit-wise contiguous block from the Class C
space allocated for its geographic region.
For instance, an European organization which requires fewer than 2048
unique IP addresses and more than 1024 would be assigned 8 contiguous
class C network numbers from the number space reserved for European
networks, 126.96.36.199 - 188.8.131.52. If an organization from
Central America required fewer than 512 unique IP addresses and more
than 256, it would receive 2 contiguous class C network numbers from
the number space reserved for Central/South American networks,
184.108.40.206 - 220.127.116.11.
The IR or the registry to whom the IR has delegated the registration
function will determine the number of Class C network numbers to
assign to a network subscriber based on the following criteria:
1) requires fewer than 256 addresses 1 class C network
2) requires fewer than 512 addresses 2 contiguous class C networks
3) requires fewer than 1024 addresses 4 contiguous class C networks
4) requires fewer than 2048 addresses 8 contiguous class C networks
5) requires fewer than 4096 addresses 16 contiguous class C networks
The number of addresses that a network subscriber indicates that it
needs should be based on a 24 month projection.
The maximal block of class C nets that should be assigned to a
subscriber consists of sixteen contiguous class C networks which
corresponds to a single IP prefix the length of which is twelve bits.
If a subscriber has a requirement for more than 4096 unique IP
addresses it should most likely receive a Class B net number.
This proliferation of class C network numbers may aid in preserving
the scarcity of class A and B numbers, but it is sure to accelerate
the explosion of routing information carried by Internet routers.
Inherent in these recommendations is the assumption that there will
be modifications in the technology to support the larger number of
network address assignments due to the decrease in assignments of
Class A and B numbers and the proliferation of Class C assignments.
Many proposals have been made to address the rapid growth of network
assignments and a discussion of those proposals is beyond the scope
and intent of this paper.
These recommendations for management of the current IP network number
space only profess to delay depletion of the IP address space, not to
postpone it indefinitely.
The author would like to acknowledge the substantial contributions
made by the members of the following two groups, the Federal
Engineering Planning Group (FEPG) and the International Engineering
Planning Group (IEPG). This document also reflects many concepts
expressed at the IETF Addressing BOF which took place in Cambridge,
MA in July 1992. In addition, Jon Postel (ISI) and Yakov Rekhter
(T.J. Watson Research Center, IBM Corp.) reviewed this document and
contributed to its content. The author thanks those groups and
individuals who have been sighted for their comments.
 Wang, Z., and J. Crowcroft, "A Two-Tier Address Structure for the
Internet: A Solution to the Problem of Address Space Exhaustion",
RFC 1335, University College London, May 1992.
 "Internet Domain Survey", Network Information Systems Center, SRI
International, July 1992.
 Ford, P., "Working Draft - dated 6 May 1992", Work in Progress.
 Solensky F., and F. Kastenholz, "A Revision to IP Address
Classifications", Work in Progress, March 1992.
 Fuller, V., Li, T., Yu, J., and K. Varadha, "Supernetting: an
Address Assignments and Aggregation Strategy", RFC 1338, BARRNet,
cisco, Merit, OARnet, June 1992.
 Rekhter, Y., and T. Li, "Guidelines for IP Address Allocation",
Work in Progress, August 1992.
 Cerf, V., "IAB Recommended Policy on Distributing Internet
Identifier Assignment and IAB Recommended Policy Change to
Internet 'Connected' Status", RFC 1174, CNRI, August 1990.
Security issues are not discussed in this memo.
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