Internet Standard Subnetting Procedure
RFC 950

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Network Working Group                                J. Mogul (Stanford)
Request for Comments: 950                                J. Postel (ISI)
                                                             August 1985

                 Internet Standard Subnetting Procedure

Status Of This Memo

   This RFC specifies a protocol for the ARPA-Internet community.  If
   subnetting is implemented it is strongly recommended that these
   procedures be followed.  Distribution of this memo is unlimited.

Overview

   This memo discusses the utility of "subnets" of Internet networks,
   which are logically visible sub-sections of a single Internet
   network.  For administrative or technical reasons, many organizations
   have chosen to divide one Internet network into several subnets,
   instead of acquiring a set of Internet network numbers.  This memo
   specifies procedures for the use of subnets.  These procedures are
   for hosts (e.g., workstations).  The procedures used in and between
   subnet gateways are not fully described.  Important motivation and
   background information for a subnetting standard is provided in
   RFC-940 [7].

Acknowledgment

   This memo is based on RFC-917 [1].  Many people contributed to the
   development of the concepts described here.  J. Noel Chiappa, Chris
   Kent, and Tim Mann, in particular, provided important suggestions.
   Additional contributions in shaping this memo were made by Zaw-Sing
   Su, Mike Karels, and the Gateway Algorithms and Data Structures Task
   Force (GADS).

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RFC 950                                                      August 1985
Internet Standard Subnetting Procedure

1.  Motivation

   The original view of the Internet universe was a two-level hierarchy:
   the top level the Internet as a whole, and the level below it
   individual networks, each with its own network number.  The Internet
   does not have a hierarchical topology, rather the interpretation of
   addresses is hierarchical.  In this two-level model, each host sees
   its network as a single entity; that is, the network may be treated
   as a "black box" to which a set of hosts is connected.

   While this view has proved simple and powerful, a number of
   organizations have found it inadequate, and have added a third level
   to the interpretation of Internet addresses.  In this view, a given
   Internet network is divided into a collection of subnets.

   The three-level model is useful in networks belonging to moderately
   large organizations (e.g., Universities or companies with more than
   one building), where it is often necessary to use more than one LAN
   cable to cover a "local area".  Each LAN may then be treated as a
   subnet.

   There are several reasons why an organization might use more than one
   cable to cover a campus:

      - Different technologies:  Especially in a research environment,
        there may be more than one kind of LAN in use; e.g., an
        organization may have some equipment that supports Ethernet, and
        some that supports a ring network.

      - Limits of technologies:  Most LAN technologies impose limits,
        based on electrical parameters, on the number of hosts
        connected, and on the total length of the cable.  It is easy to
        exceed these limits, especially those on cable length.

      - Network congestion:  It is possible for a small subset of the
        hosts on a LAN to monopolize most of the bandwidth.  A common
        solution to this problem is to divide the hosts into cliques of
        high mutual communication, and put these cliques on separate
        cables.

      - Point-to-Point links:  Sometimes a "local area", such as a
        university campus, is split into two locations too far apart to
        connect using the preferred LAN technology.  In this case,
        high-speed point-to-point links might connect several LANs.

   An organization that has been forced to use more than one LAN has
   three choices for assigning Internet addresses:

Mogul & Postel                                                  [Page 2]



RFC 950                                                      August 1985
Internet Standard Subnetting Procedure

      1. Acquire a distinct Internet network number for each cable;
         subnets are not used at all.

      2. Use a single network number for the entire organization, but
         assign host numbers without regard to which LAN a host is on
         ("transparent subnets").

      3. Use a single network number, and partition the host address
         space by assigning subnet numbers to the LANs ("explicit
         subnets").

   Each of these approaches has disadvantages.  The first, although not
   requiring any new or modified protocols, results in an explosion in
   the size of Internet routing tables.  Information about the internal
   details of local connectivity is propagated everywhere, although it