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Network Working Group                                     David D. ClarkRequest for Comments: 932                                       MIT, LCS                                                            January 1985A SUBNETWORK ADDRESSING SCHEMESTATUS 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 hisRFC-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. (SeeRFC-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 1985A 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 aClark                                                           [Page 2]

RFC 932                                                     January 1985A 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 subnetworkedClark                                                           [Page 3]

RFC 932                                                     January 1985A 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.Clark                                                           [Page 4]