RFC 932 (rfc932) - Page 2 of 4

Subnetwork addressing scheme

RFC 932                                                     January 1985
A Subnetwork Addressing Scheme


   The gateways and hosts attached to this subnetted network use these
   addresses as ordinary Class C addresses.  Thus, no modification to
   any host software is required for hosts attached to a subnetwork.

   For gateways not directly attached to the subnetted network, it is an
   unacceptable burden to separately store the routing information to
   each of the subnets. The goal of any subnet addressing scheme is to
   provide a strategy by which distant gateways can store routing
   information for the network as a whole.  In this scheme, since the
   first two bytes of the address is the same for every subnet in the
   network, those first two bytes can be stored and manipulated as if
   they are a single Class B address by a distant gateway. These
   addresses, which can be used either as a Class B or Class C address
   as appropriate, have been informally called Class "B 1/2" addresses.

   In more detail, a gateway would treat Class C addresses as follows
   under the scheme.  First, test to see whether the high order bit of
   the address is on.  If not, the address is an ordinary Class C
   address and should be treated as such.

   If the bit is on, this Class C address identifies a subnet of a
   network.  Test to see if this gateway is attached to that network.
   If so, treat the address as an ordinary Class C address.

   If the gateway is not attached to the network containing that
   subnetwork, discard the third byte of the Class C address and treat
   the resulting two bytes as a Class B address.  Note that there can be
   no conflict between this two-byte pattern and an ordinary Class B
   address, because the first bits of this address are not those of a
   valid Class B address, but rather those of a Class C address.

OPTIMIZATIONS

   If a network grows to more than 256 subnetworks, it will be necessary
   to design two distinct blocks of special Class C addresses, and to
   view this aggregate as two separate networks.  However, the gateways
   of these two networks can, by proper design, run a joint routing
   algorithm which maintains optimal routes between the two halves, even
   if they are connected together by a number of gateways.

   Indeed, in general it is possible for gateways that are not directly
   attached to a subnetworked network to be specially programmed to
   remember the individual Class C addresses, if doing so provides
   greatly improved network efficiency in some particular case.

   It was stated earlier that no modification to the host software is
   necessary to implement this scheme.  There is one case in which a


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