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+Network Working Group                                     David D. Clark
+Request for Comments: 932                                       MIT, LCS
+                                                            January 1985
+
+                     A SUBNETWORK ADDRESSING SCHEME
+
+
+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.
+
+INTRODUCTION
+
+   Several recent RFCs have discussed the need for a "subnet" structure
+   within the internet addressing scheme, and have proposed strategies
+   for "subnetwork" addressing and routing.  In particular, Jeff Mogul
+   in his RFC-917, "Internet Subnets", describes an addressing scheme in
+   which a variable number of the leading bits of the host portion of
+   the address are used to identify the subnet.  The drawback to this
+   scheme is that it is necessary to modify the host implementation in
+   order to implement it.  While the modification is a simple one, it is
+   necessary to retrofit it into all implementations, including those
+   which are already in the field. (See RFC-917 by Mogul for various
+   alternative approaches to this problem, such as using Address
+   Resolution Protocol.)
+
+   This RFC proposes an alternative addressing scheme for subnets which,
+   in most cases, requires no modification to host software whatsoever.
+   The drawbacks of this scheme are that the total number of subnets in
+   any one network are limited, and that modification is required to all
+   gateways.
+
+THE PROPOSAL
+
+   In this scheme, the individual subnets of a network are numbered
+   using Class C addresses.  Since it is necessary with this scheme that
+   a Class C address used to number a subnet be distinguishable from a
+   Class C address used to number an isolated network, we will reserve
+   for subnetworks the upper half of the Class C address space, in other
+   words all those Class C addresses for which the high order bit is on.
+   When a network is to be organized as a series of subnetworks, a block
+   of these reserved Class C addresses will be assigned to that network,
+   specifically a block of 256 addresses having the two first bytes
+   identical.  Thus, the various subnetworks of a network are
+   distinguished by the third byte of the Internet address.  (This
+   addressing scheme implies the limitation that there can only be 256
+   subnetworks in a net.  If more networks are required, two blocks will
+   have to be allocated, and the total viewed as two separate networks.)
+
+
+
+Clark                                                           [Page 1]
+
+
+
+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
+
+
+Clark                                                           [Page 2]
+
+
+
+RFC 932                                                     January 1985
+A Subnetwork Addressing Scheme
+
+
+   minor modification may prove helpful.  Consider the case of a distant
+   host, not immediately attached to this subnetworked network.  That
+   host, even though at a distance, will nonetheless maintain separate
+   routing entries for each of the distinct subnetwork addresses about
+   which it has any knowledge.  For most hosts, storing this information
+   for each subnet represents no problem, because most implementations
+   do not try to remember routing information about every network
+   address in the Internet, but only those addresses that are of current
+   interest.  If, however, for some reason the host has a table which
+   attempts to remember routing information about every Internet address
+   it has ever seen, than that host should be programmed to understand
+   the gateway's algorithm for collapsing the addresses of distant
+   subnets from three bytes to two.  However, it is not a recommended
+   implementation strategy for the host to maintain this degree of
+   routing information, so under normal circumstances, the host need not
+   be concerned with the C to B conversion.
+
+DRAWBACK
+
+   The major drawback of this scheme is that any implementation storing
+   large tables of addresses must be changed to know the "B 1/2"
+   conversion rule. Most importantly, all gateways must be programmed to
+   know this rule.  Thus, adoption of this scheme will require a
+   scheduled mandatory change by every gateway implementation.  The
+   difficulty of organizing this is unknown.
+
+OTHER VARIATIONS
+
+   It is possible to imagine other variations on the patterns of
+   collapsing addresses.  For example, 256 Class B addresses could be
+   gathered together and collapsed into one Class A address.  However,
+   since the first three bits of the resulting Class A address would be
+   constrained, this would permit only 32 such subnetted networks to
+   exist.  A more interesting alternative would be to permit the
+   collapse of Class C addresses into a single Class A address.  It is
+   not entirely obvious the best way of organizing the sub-fields of
+   this address, but this combination would permit a few very large nets
+   of subnets to be assembled within the Internet.
+
+   The most interesting variation of "B 1/2" addresses is to increase
+   the number of bits used to identify the subnet by taking bits from
+   the resulting Class B address.  For example, if 10 bits were used to
+   identify the subnet (providing 1024 subnets per network), then the
+   gateway, when forming the equivalent address, would not only drop the
+   third byte but also mask the last two bits of the B address.  Since
+   the first three bits of the address are constrained, this would leave
+   13 bits for the network number, or 8192 possible subnetworked
+
+
+Clark                                                           [Page 3]
+
+
+
+RFC 932                                                     January 1985
+A Subnetwork Addressing Scheme
+
+
+   networks.  This number is not as large as would be desirable, so it
+   is clear that selecting the size of the subnet field is an important
+   compromise.
+
+   Danny Cohen has suggested that this scheme should be fully
+   generalized so that the boundaries between the network, subnetwork,
+   and host field be arbitrarily movable.  The problem in such a
+   generalization is to determine how the gateway is to maintain the
+   table or algorithm which permits the collapsing of the address to
+   occur.  This RFC proposes that, in the short run, only one single
+   form of "B 1/2" addresses be implemented as an Internet subnet
+   standard.
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+Clark                                                           [Page 4]
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