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