On the assignment of subnet numbers (RFC1219)
Original Publication Date: 1991-Apr-01
Included in the Prior Art Database: 2000-Sep-12
Internet Society Requests For Comment (RFCs)
Status Of This Memo
Network Working Group P. Tsuchiya
Request for Comments: 1219 Bellcore
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
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
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 thei...