On the assignment of subnet numbers (RFC1219)
Original Publication Date: 1991-Apr-01
Included in the Prior Art Database: 2019-Feb-11
Internet Society Requests For Comment (RFCs)
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. This memo provides information for the Internet community. It does not specify an Internet standard.
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.
RFC-950  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 . The Gateway Requirements RFC  settled the issue in favor of allowing different masks, and therefore future routing protocols may be expected to support this feature; OSPF  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.
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.
The Subnetting standard,...