RFC 1219 (rfc1219) - Page 2 of 13

On the assignment of subnet numbers

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



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