RFC 917 (rfc917) - Page 2 of 22

Internet subnets

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".)


Mogul