Request for Comments: 1366
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 unlimited.
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:
- Qualifications for Distributed Regional Registries
- Allocation of the Network Number Space by the Internet Registry
- 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 numbers.
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 allocation divisions.
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 18.104.22.168 through 22.214.171.124. 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 - 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 - 184.108.40.206
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 follows:
Multi-regional 192.0.0.0 - 220.127.116.11 Europe 18.104.22.168 - 22.214.171.124 Others 126.96.36.199 - 188.8.131.52 North America 184.108.40.206 - 220.127.116.11 Central/South America 18.104.22.168 - 22.214.171.124 Pacific Rim 126.96.36.199 - 188.8.131.52 Others 184.108.40.206 - 220.127.116.11 Others 18.104.22.168 - 22.214.171.124
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 126.96.36.199 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:
- the organization presents a subnetting plan which documents more than 32 subnets within its organizational network
- 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 basis.
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, 188.8.131.52 - 184.108.40.206. 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, 220.127.116.11 - 18.104.22.168.
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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