RFC 936 (rfc936) - Page 2 of 4

Another Internet subnet addressing scheme

RFC 936                                                    February 1985
Another Internet Subnet Addressing Scheme


   at the time each network interface's address is set at boot time.)
   For local addresses, the host part is examined for a subnet number.
   Local addresses may be on the main network, or they may be on a
   subnet.  The high-order bit of the host number is used to distinguish
   between subnets and the main net.  If the high-order bit of the host
   field is set, then the remainder of the high-order byte of the host
   part is taken to be the subnet number.  If the high-order bit is
   clear, then the address is interpreted in the normal fashion.  For
   Class A networks, using 8-bit subnet fields, this allows a network
   with up to 127 subnets, each of 65535 hosts maximum, and a main net
   with 2^23 hosts.  Class B nets may include 127 subnets, each of up to
   255 hosts, and 32767 hosts on the main net.  Class C networks are not
   currently included in this scheme. They might be reasonably be added,
   using four bits of the host part for a subnet desgination and four
   bits for the host, allowing 8 subnets of 15 hosts and 126 hosts on
   the main net.

   The current implementation does not use subnet numbers separately
   from the network field, but instead treats the subnet field as an
   extension of the network field.  Functions that previously returned
   the network number from an address now return a network or
   network-subnetwork number.  Conveniently, Class A subnets are
   distinguishable from Class B networks, although each is a 16-bit
   quantity, and Class B subnets are disjoint with Class C network
   numbers.  The net result is that subnets appear to be separate,
   independent networks with their own routing entries within the
   network, but outside of the network, they are invisible.  There is no
   current facility at Berkeley for broadcasting on the logical network;
   broadcasting may be done on each subnet that uses harware capable of
   broadcast.

Discussion

   There have been several earlier proposals for methods of allowing
   several physical networks to share an Internet network designation,
   and to provide routing within this logical network.  RFC-917 proposes
   a means for encoding the host part of each local address such that
   the hosts, or the gateways connecting them, are able to determine the
   physical network for the host.  The current proposal is most similar
   to that scheme; the differences are discussed in detail below.

   Another proposal (RFC-925) involves the use of intelligent gateways
   to perform routing for unmodified hosts, using an Address Resolution
   Protocol (ARP) [2].  This has the advantage of placing all
   modifications in the gateways, but is likely to require additional
   routing protocols and caching mechanisms in the gateways in order to
   avoid excessive broadcasts for address resolution.  A modification of


Karels