Network Working Group F. Solensky INTERNET-DRAFT F. Kastenholz Updates: 791, 1122 Clearpoint Research Corp. November, 1991 A Revision to IP Address Classifications Status of this Memo This Internet Draft document will be submitted to the RFC editor as a standards document. Comments and suggestions are welcome and may be sent to the authors. Distribution of this memo is unlimited. Abstract This memo presents an extension to the method of classifying and assigning IP network numbers. It is intended to provide a work- around to the imminent exhaustion of assignable Class B network numbers by defining the format of a new Class E address, allocated using the current 'reserved' address class, and refresh that reserve by creating two new reserved address classes out of unused portions of the old Class A and C numbering space. It is a product of a "birds-of-a-feather" (BOF) discussion held on July 31, 1991 at the twenty-first IETF conference in Atlanta, GA. It should be noted that this document does NOT address the limitations inherent in the current routing architectures and technology [1]. These must wait until new architectures are developed. Specifically, the issue of scaling is not addressed. Background During the latter part of the 1980's, an ever-increasing number of organizations came to realize the advantage and importance of allowing their computer systems to interconnect with other systems and networks around the globe. This has both caused and reinforced the tremendous growth in the size of the Internet during this period. While this is usually seen as a positive trend, it has not been without its drawbacks. One of the more immediate problems that this sudden growth has presented is a continuing heavy demand for Class B network numbers. Of the three classes of IP network numbers, A (up to almost 2^24 hosts in the 'rest' portion of the address), B (up to 2^16), and C (up to 2^8), the class B numbers are being used up at the highest rate. While there are still a very large number of Class C addresses Solensky, Kastenholz [Page 1] INTERNET DRAFT November, 1991 available, few organizations expect that their connectivity needs will be satisfied within the limitations of 254 IP addresses, particularly if subnetting is being used. The level of demand for Class B address assignments can be illustrated by a short analysis of the data available. In the period between July 1990 and September 1991, the number of assigned Class B network numbers grew from 2533 to 6239 [2,3]; the latter figure represents a usage of just over one third (38.1% to be exact) of the total available Class B network numbers. This increase averages out to an annual growth rate of over 116%. If this trend were to continue, the pool of available Class B network numbers would be depleted by December 1992. While the authors acknowledge that a logistic or "s-shaped" curve would be a more realistic model, a projection based on this assmption would not be realistic until we have clearly passed the inflection point on the curve - the point at which the curve starts to climb less rapidly towards its upper limit. The data available at this time suggests that this leveling off has not yet occured: the annual growth rate in the allocation of Class B network numbers between 1983 and mid-1990 was only 78% [4]. Whatever the exact shape of the curve, the conclusion that severe problems will erupt shortly, with the exhaustion of the Class B network numbers, is inescapable. The obvious corollary is that a short-term fix, until the more fundamental problems referred to above can be solved, is necessary. This document contains that fix. Class E Network Numbers The entire current reserved address space (prefix 1111) will become Class E address space and will be used for the assignment of new Class E IP network numbers. Within the 28 bits available in Class E addresses, the first sixteen will define the network number and the remaining twelve will be the local address, as illustrated below. 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1 1 1 1| NETWORK | Local Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Class E address The network number 255.255.240.0 is reserved so that it does not conflict with the address reserved for IP broadcasts (255.255.255.255). It was felt that splitting the network and local address fields into these sizes met some of the more important design objectives: Solensky, Kastenholz [Page 2] INTERNET DRAFT November, 1991 * The number of networks created by this division - over 65,000 - should be sufficient to meet the needs of the immediate future while other long-term solutions are being developed. The alter- native of using fewer bits in the network portion of the address (including 2^12 more Class B-sized networks) had been considered but generally rejected since it would allow proportionally less time to develop and deploy a replacement Internet architecture. * Many sites that are currently requesting Class B numbers do not need a whole Class B space, but could easily use something a little smaller. The size of a local network in this address class - 4094 hosts in an unsubnetted environment - is large enough to be useful to many organizations without being so large that it becomes sparsely populated. It also provides a local field large enough to keep subnetting practical. * The creation of this new address class should sufficiently reduce the demand for the remaining class B network numbers so that their assignment can be limited to larger sites. Another benefit of this division, while not of great import but nevertheless noteworthy, is that it keeps the division of the network and local addresses fields on nybble boundaries and thereby easier to pick out the individual fields when displayed in hexadecimal nota- tion. The proposal to continue the current practice of allocating a space whose prefix started with all 1's and ended with a 0 (i.e. allocate 11110 and reserve 11111) had also been considered. The problem with doing so, however, is that this practice demonstrates the law of diminishing returns: the processing overhead of separating any IP address into its network and local address fields gets increasingly complex while shrinking the reserved address space into a less useful portion of the total. Another alternative that was discussed was to use the upper half of the Class C address space (prefix 1101 rather than 1111) in the same manner. This option needs to factor in the impact this would have on routers that would interpret these Class E addresses as if they were members of the same Class C network as opposed to discarding them as invalid IP addresses. While it is true that older routers would still attempt to forward packets in this part of the address space, an argument can be made that this is, in fact, not desirable. Even if the router were able to handle them as if they were disjoint or independent subnetworks, it would soon get bogged down trying to keep track of all of these networks. Further, unless the new numbers were allocated from the reserved space (and thus dropped by unmodified software), there would be less impetus on people's part to change Solensky, Kastenholz [Page 3] INTERNET DRAFT November, 1991 their software. The ability to model a Class E net as a group of Class C's would, from the larger viewpoint, compound the scaling problem that will soon make a new IP architecture necessary. Revisions to IP Address Classes A and C. Since the allocation of the remaining reserved IP address space to Class E addresses uses the final block of reserved numbers, it becomes necessary to create a new reserved space. The rationale behind doing so is to allocate sufficient numbering space for the development of alternative network number classifications (including the restoration of their prior interpretations) should the need arise in the near future. By doing so at this time would provide enough of a lead-time to allow the use of the old meaning to disappear from the net before the new use was taken up. The space for both Class A and C network numbers will be reduced to half their current size. The low half of these address ranges - net- work number fields starting with "0" - will continue to be used in their present form, as illustrated. 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0| NETWORK | Local Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Revised Class A address 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1 1 0 0| NETWORK | Local Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Revised Class C address This reduces the amount of assignable network numbers in these classes to 63 and 1,048,576, respectively. It should be noted, how- ever, the demand for assigned numbers within these network classes has not been nearly as great as that for Class B. Reserved Addresses The upper half of the old class A and C address space will be redesignated as classes A-sharp and C-sharp respectively* and are illustrated below. The Class A-sharp addresses in the range of 127.0.0.0 through 127.255.255.255 will be reserved to allow their _________________________ Solensky, Kastenholz [Page 4] INTERNET DRAFT November, 1991 continued use as an IP loopback address. 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 1| reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Class A-sharp address 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1 1 0 1| reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Class C-sharp address The size of the A-sharp space is particuarly advantageous: if it turns out that the demand for class B addresses remains high in spite of the alternative class E network numbers, the A-sharp space could be defined to have the same network/local address composition as class B addresses. The class C-sharp numbers are also reserved for the purpose of having a portion of the IP address space available for future development efforts. The address space that this opens up is exactly the same size of the current Class E address space. At current growth rates, it is very unlikely that there will be any short-term depeletion of assignable class C numbers. The software changes needed to recognize these two reserved address classes would be a minor incremental effort above that necessary for supporting class E addresses. By advocating the change at this time, it prevents the aggravation that may later exist if two different interpretions of the same address were to coexist in the future. For the sake of completeness, Class B and D addresses are also illus- trated here. The use of Class D or multicast addresses is specified _________________________ * The musically inclined may appreciate the mnemonic device: the two address classes that are likely to become fully utilized correspond to the white keys on a piano that do not have black keys a half- step above them: B and E. However, one needs to be careful not to read too much into these names since, as stated earlier, this methodology does not address the issue of scaling. Solensky, Kastenholz [Page 5] INTERNET DRAFT November, 1991 in [5]. 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1 0| NETWORK | Local Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Class B address 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1 1 1 0| multicast address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Class D address A portion of the Class D (multicast) space could eventually be reserved. It does not seem as desirable an option at this time, since it would lead to use of a 5 bit format prefix (currently, the longest one is 4 bits) and would save only a small portion (1/32nd) of the address space. Multicast addresses will, however, only be allocated from the low part of the Class D space to allow reuse of this space if circumstances become desperate. Conclusions It must be emphasized that this is intended only to be a work-around to the problem. It is by no means a solution. While it defines a network classification that is four times the size of the original Class B space, this will only survive only two years if current growth rates continue. By that time, it is expected that the increased amount of network connectivity which has been exhibiting similar growth rates [4,6] will cause the computational intensity of keeping track of these routes to require an entirely different rout- ing and addressing architecture for the Internet such as the one described in [1]. This change also points out the necessity of having hosts not pry into address formats. It is plausible to deploy a new network number format if only the routers have to be changed; doing so in a world where most types of host software have to be changed as well is clearly problematic. References: [1] "The IP Addressing Issue", J. Noel Chiappa, Internet Draft, October, 1990. Solensky, Kastenholz [Page 6] INTERNET DRAFT November, 1991 [2] "Internet Numbers", S. Kirkpatrick, M. Stahl, M. Recker, RFC 1166, SRI International, July 1990. [3] Internet Monthly Report, A. Westine [ed], September, 1991. [4] "Continued Internet Growth", Frank Solensky, Proceedings of the Eighteenth Internet Engineering Task Force, July 30-August 3, 1990. pages 59-61. [5] "Host Extentions for IP Multicasting", S. Deering, RFC 1112, SRI International, August 1989. [6] "Growth of the Internet", Mike St. Johns, Proceedings of the Thir- teenth Internet Engineering Task Force, April 11-14, 1989, pages 244-248. Authors' Address: Frank Solensky Frank Kastenholz Clearpoint Research Corp. 35 Parkwood Drive Hopkinton, MA 01748 Phone: (508) 435-2000 Email: solensky@clearpoint.com, kasten@clearpoint.com Solensky, Kastenholz [Page 7]