RFC 940 (rfc940) - Page 2 of 3

Toward an Internet standard scheme for subnetting

RFC 940                                                       April 1985
Toward an Internet Standard Scheme for Subnetting


   This topic has been discussed in a set of RFCs [1,2,3,4] and in a
   flurry of messages in the Gateway Algorithms and Data Structures Task
   Force.  It is strongly suggested that if subnetting is used at all,
   it be according this new standard scheme.

APPROACH

   An Internet address currently consists of a two-layer hierarchy, a
   'network' and a per-network 'rest' field.  This subnet scheme adds an
   optional 'subnet' layer and field.

   The subnet field is created by stealing some bits from the rest (or
   host) field of the address.  The details of the subnet field are site
   specific.  All three classes (A, B, and C) of networks may be
   subnetted.

   The use of subnets is an optional local decision.  The fact that a
   network has subnets is invisible outside that network, and the change
   is local and can be instituted at a site without any global Internet
   perturbations.  A complex of links is assigned a single IP network
   number, and outside that complex it appears as a single network with
   that number.  Only inside does local structure appear.

   However, while the decision to use subnets at a site is optional, any
   IP implementation which may possibly be used in a potentially
   subnetted environment, should provide for subnet field configuration
   as described above.  Such an implementation will function properly in
   environments with or without subnetting.  On the other hand,
   implementations lacking this provision will not function in a
   subnetted environment, and are thus potentially less useful.

   This specifications is not intended to require a particular
   implementation technique inside the host, but rather to define the
   external behavior of the host in a subnetted environment.  It does
   not specify how routing is done or the details of host construction.
   Note that gateways are hosts, too.

   However, it seems easiest to explain the approach by describing one
   possible host implementation.

      Example Implementation:

         Let us use "subnet" to mean the locally attached transmission
         medium.

         The key decision to be made is "Is the destination IP address



GADS