Subnetwork addressing scheme

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.)

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

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

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.