On the assignment of subnet numbers
RFC 1219
Document | Type |
RFC - Informational
(April 1991; No errata)
Was draft-tsuchiya-subnetnos (individual)
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Author | Paul Tsuchiya | ||
Last updated | 2013-03-02 | ||
Stream | Legacy | ||
Formats | plain text html pdf htmlized bibtex | ||
Stream | Legacy state | (None) | |
Consensus Boilerplate | Unknown | ||
RFC Editor Note | (None) | ||
IESG | IESG state | RFC 1219 (Informational) | |
Telechat date | |||
Responsible AD | (None) | ||
Send notices to | (None) |
Network Working Group P. Tsuchiya Request for Comments: 1219 Bellcore April 1991 On the Assignment of Subnet Numbers Status Of This Memo This memo suggests a new procedure for assigning subnet numbers. Use of this assignment technique within a network would be a purely local matter, and would not effect other networks. Therefore, the use of these procedures is entirely discretionary. This memo provides information for the Internet community. It does not specify an Internet standard. Distribution of this memo is unlimited. Overview RFC-950 [2] specifies a procedure for subnetting Internet addresses using a bit-mask. While RFC-950 allows the "ones" in the subnet mask to be non-contiguous, RFC-950 recommends that 1) they be contiguous, and 2) that they occupy the most significant bits of the "host" part of the internet address. RFC-950 did not specify whether different subnets of the same network may have different masks. This ambiguity was unfortunate, as it resulted in development of routing protocols that do not support different masks; see e.g., RIP [6]. The Gateway Requirements RFC [7] settled the issue in favor of allowing different masks, and therefore future routing protocols may be expected to support this feature; OSPF [3] is an example. The network administrator must of course determine the mask for each subnet. This involves making an estimate of how many hosts each subnet is expected to have. As it is often impossible to predict how large each subnet will grow, inefficient choices are often made, with some subnets under-utilized, and others possibly requiring renumbering because of exceeded capacity. This memo specifies a procedure for assigning subnet numbers that eliminates the need to estimate subnet size. Essentially, host bits (mask = 0) are assigned from the least significant bit working towards the most, and subnet bits (mask = 1) are assigned from the most significant bit working towards the least. As subnets grow, more host bits are assigned. As the number of subnets grows, more subnet bits are assigned. While this process does sometimes result Tsuchiya [Page 1] RFC 1219 On the Assignment of Subnet Numbers April 1991 in new subnet masks, no host ever need change addresses. This technique is not new, but it is also not widely known, and even less widely implemented. With the development of new routing protocols such as OSPF, it is possible to take full advantage of this technique. The purpose of this memo, then, is to make this technique widely known, and to specify it exactly. This memo requires no changes to existing Internet standards. It does, however, require that the intra-domain routing protocol handle multiple different subnet masks. Acknowledgments The author would like to thank Phil Karn, Charles Lynn, Jeff Mogul, and Charles Wolverton for their helpful suggestions. Special thanks go to Joel Halpern for his painstaking debugging of the detailed specification and the examples. 1. Motivation The Subnetting standard, RFC-950, specifies that the Host part of the formally 2-level Internet address can be divided into two fields, Subnet and Host. This gives the Internet address a third level of hierarchy, and the concomitant firewalls and savings in routing overhead. It also introduces increased inefficiency in the allocation of addresses. This inefficiency arises from the fact that the network administrator typically over-estimates the size (number of hosts) of any single subnetwork, in order to prevent future re-addressing of subnets. It may also occur if the routing protocol being used does not handle different length subnets, and the administrator must therefore give every subnet an amount of space equivalent to that received by the largest subnet. (This RFC does not help in the latter case, as the technique herein requires different length subnets.) The administrative hassle associated with changing the subnet structure of a network can be considerable. For instance, consider the following case. A network has three subnets A, B, and C. Assume that the lowest significant byte is the host part, and the next byte is the subnet part (that is, the mask is 255.255.255.0). Assume further that A has subnet 1.0, B has subnet 2.0, and C has subnet 3.0. Now, assume that B grows beyond its allocation of 254 hosts. Ideally, we would like to simply change B's mask without changing any of the host addresses in B. However, the subnets numerically above Tsuchiya [Page 2]Show full document text