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.
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
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
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.)
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.
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
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.
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.
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
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
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