Internet subnets
RFC 917

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Network Working Group                                      Jeffrey Mogul
Request for Comments: 917                    Computer Science Department
                                                     Stanford University
                                                            October 1984

                            INTERNET SUBNETS

Status Of This Memo

   This RFC suggests a proposed protocol for the ARPA-Internet
   community, and requests discussion and suggestions for improvements.
   Distribution of this memo is unlimited.

Overview

   We discuss 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.

   We propose procedures for the use of subnets, and discuss approaches
   to solving the problems that arise, particularly that of routing.

Acknowledgment

   This proposal is the result of discussion with several other people.
   J. Noel Chiappa, Chris Kent, and Tim Mann, in particular, provided
   important suggestions.

1. Introduction

   The original view of the Internet universe was a two-level hierarchy:
   the top level the catenet as a whole, and the level below it a
   collection of "Internet Networks", each with its own Network Number.
   (We do not mean that the Internet has a hierarchical topology, but
   that the interpretation of addresses is hierarchical.)

   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 might (or might not) be divided into a collection of
   subnets.

   The original, two-level, view carries a strong presumption that, to a
   host on an Internet network, that network may be viewed as a single
   edge; to put it another way, the network may be treated as a "black
   box" to which a set of hosts is connected.  This is true of the

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RFC 917                                                     October 1984
Internet Subnets

   ARPANET, because the IMPs mask the use of specific links in that
   network.  It is also true of most local area network (LAN)
   technologies, such as Ethernet or ring networks.

   However, this presumption fails in many practical cases, because in
   moderately large organizations (e.g., Universities or companies with
   more than one building) it is often necessary to use more than one
   LAN cable to cover a "local area".  For example, at this writing
   there are eighteen such cables in use at Stanford University, with
   more planned.

   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 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:

      1. Acquire a distinct Internet network number for each cable.

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

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

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RFC 917                                                     October 1984
Internet Subnets

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