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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.)

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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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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

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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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