RFC 1219 (rfc1219) - Page 2 of 13
On the assignment of subnet numbers
Alternative Format: Original Text Document
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
