RFC 1219 (rfc1219) - Page 1 of 13

On the assignment of subnet numbers

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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.

Overview

   RFC-950 [2] 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 [6].  The Gateway Requirements RFC [7]
   settled the issue in favor of allowing different masks, and therefore
   future routing protocols may be expected to support this feature;
   OSPF [3] 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
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